docs: document system-level backlog and out-of-git operation

README.md

@@ -9,8 +9,8 @@ Capture and manage ideas from the command line. A worktree-aware backlog tracker
 ## Why idea?
 
 - **Plain Markdown, not a database** — your backlog is a checked-in `fab/backlog.md` file. Hand-edit it, grep it, diff it, review it in PRs. `idea` is one (canonical) writer of the format; the file is the contract.
-- **Per-repo, not global** — every repo has its own backlog. No central app to log into, no cross-project noise.
-- **Worktree-aware** — by default `idea` reads/writes the *current* worktree's backlog, so parallel changes don't step on each other. `--main` opts into the shared backlog when you need it.
+- **Per-repo by default, with a global escape hatch** — every repo has its own backlog, so there's no cross-project noise. When you're not in a repo (or want a personal, cross-cutting list), `idea` falls back to a system-level backlog at `~/.config/idea/backlog.md` — reachable from anywhere with `--system`.
+- **Worktree-aware** — by default `idea` reads/writes the *current* worktree's backlog, so parallel changes don't step on each other. `--main` opts into the shared backlog, and `--system` targets the global one.
 - **Short, addressable IDs** — every idea gets a 4-character ID like `[qu1d]` you can type into any command. Queries also match free-text substrings.
 - **Hooks into fab-kit** — `fab/backlog.md` is the same file fab-kit's `/fab-new` reads, so capturing an idea today and starting a change from it tomorrow is one command.
 
@@ -101,10 +101,12 @@ This is the one behavior worth knowing in detail. `idea` resolves the backlog fi
 | Main repo | `idea add "..."` | `<main>/fab/backlog.md` |
 | Linked worktree | `idea add "..."` | `<worktree>/fab/backlog.md` (local to this worktree) |
 | Linked worktree | `idea --main add "..."` | `<main>/fab/backlog.md` (shared with the team) |
-| Anywhere | `idea --file path/to/file.md ...` | that file (relative to git root) |
-| Anywhere | `IDEAS_FILE=... idea ...` | the env-var path |
+| Outside any git repo | `idea add "..."` | `~/.config/idea/backlog.md` (the system backlog — automatic fallback) |
+| Anywhere | `idea --system add "..."` | `~/.config/idea/backlog.md` (the system backlog, even inside a repo) |
+| Anywhere | `idea --file path/to/file.md ...` | that file (relative to the git root, or to `~/.config/idea` outside a repo; absolute paths as-is) |
+| Anywhere | `IDEAS_FILE=... idea ...` | the env-var path (same rooting as `--file`) |
 
-Why the default favors the current worktree: when you're heads-down on a change and capture a thought, you usually mean "for *this* branch." `--main` is the explicit opt-in for "add this to the shared roadmap." In the main worktree, both behave identically.
+Why the default favors the current worktree: when you're heads-down on a change and capture a thought, you usually mean "for *this* branch." `--main` is the explicit opt-in for "add this to the shared roadmap." In the main worktree, `--main` and the default behave identically. `--system` is the opt-in for a personal, cross-repo list — and it's also what you get automatically when you run `idea` outside any git repo, so capture works everywhere. (The system path honors `$XDG_CONFIG_HOME`, defaulting to `~/.config/idea/backlog.md`. `--system` and `--main` are mutually exclusive.)
 
 ## Integration with fab-kit
 

docs/site/workflows.md

@@ -9,37 +9,49 @@ New here? Start with the [install guide](install.md), then come back.
 
 ## Worktree-aware resolution
 
-`idea` resolves the backlog file based on *where you run it* — not on an
-environment default or a global config. By default it reads and writes the
-**current worktree's** `fab/backlog.md`.
+`idea` resolves the backlog file based on *where you run it*. Inside a git repo
+it reads and writes the **current worktree's** `fab/backlog.md` by default —
+predictable from your directory, never from stray environment state. Outside a
+repo (or with `--system` anywhere), it uses a **system-level backlog** at
+`~/.config/idea/backlog.md`, so capture works even where there's no repo.
 
 | Where you are | What you type | Which file `idea` touches |
 |---------------|---------------|----------------------------|
 | Main repo | `idea add "..."` | `<main>/fab/backlog.md` |
 | Linked worktree | `idea add "..."` | `<worktree>/fab/backlog.md` (local to this worktree) |
 | Linked worktree | `idea --main add "..."` | `<main>/fab/backlog.md` (shared with the team) |
-| Anywhere | `idea --file path/to/file.md ...` | that file (relative to the git root) |
-| Anywhere | `IDEAS_FILE=... idea ...` | the env-var path |
-
-How resolution actually works under the hood:
-
-- **Current worktree** (the default): `idea` runs `git rev-parse --show-toplevel`
-  and uses `<toplevel>/fab/backlog.md`.
+| Outside any git repo | `idea add "..."` | `~/.config/idea/backlog.md` (system backlog — automatic fallback) |
+| Anywhere | `idea --system add "..."` | `~/.config/idea/backlog.md` (system backlog, even inside a repo) |
+| Anywhere | `idea --file path/to/file.md ...` | that file (relative to the git root, or to `~/.config/idea` outside a repo) |
+| Anywhere | `IDEAS_FILE=... idea ...` | the env-var path (same rooting as `--file`) |
+
+How resolution actually works under the hood (first match wins):
+
+- **`--system`**: skips git entirely and uses the system backlog —
+  `$XDG_CONFIG_HOME/idea/backlog.md`, defaulting to `~/.config/idea/backlog.md`.
+  Reachable from anywhere, including inside a repo. Mutually exclusive with `--main`.
+- **`--file <path>` / `IDEAS_FILE`**: overrides the path. A relative value is
+  rooted at the git root inside a repo, or at `~/.config/idea` outside one; an
+  absolute value is used verbatim.
 - **`--main`**: `idea` runs `git rev-parse --path-format=absolute --git-common-dir`
   and takes that directory's parent as the main worktree root, then uses
-  `<main>/fab/backlog.md`. In the *main* worktree, `--main` and the default
-  behave identically.
-- **`--file <path>`**: overrides the path entirely (resolved relative to the git
-  root).
-- **`IDEAS_FILE`**: same override via environment variable.
+  `<main>/fab/backlog.md`. Git-only — it errors outside a repo. In the *main*
+  worktree, `--main` and the default behave identically.
+- **Current worktree** (the in-repo default): `idea` runs `git rev-parse --show-toplevel`
+  and uses `<toplevel>/fab/backlog.md`.
+- **System fallback** (the out-of-repo default): when `git rev-parse` reports
+  you're not in a repo, `idea` gracefully falls back to the system backlog
+  instead of erroring. The config directory is created on first write.
 
 **Why the default favors the current worktree.** When you're heads-down on a
 change in a linked worktree and capture a thought, you almost always mean "for
 *this* branch." Defaulting to the current worktree keeps parallel changes from
 stepping on each other's backlogs. `--main` is the explicit opt-in for "add this
-to the shared roadmap." Resolution never reads heuristics or stray environment
-state — it asks git directly, so behavior is predictable from your current
-directory alone.
+to the shared roadmap." Inside a repo, resolution never reads heuristics or stray
+environment state — it asks git directly, so behavior is predictable from your
+current directory alone. The system backlog (`--system`, or the automatic
+out-of-repo fallback) is the one deliberate exception: a single personal list for
+cross-cutting ideas and for capture wherever there's no repo at all.
 
 ## fab-kit integration loop