feat(tui): migrate lessons.db / sessions.db to os.UserConfigDir() with workspace isolation #265

Context

The runtime databases for the SIN-Code agent are currently written to
project-relative paths:

• cmd/sin-code/tui/.sin-code/lessons.db (via TUI agent_runner.go:160)
• cmd/sin-code/tui/.sin-code/sessions.db (via TUI agent_runner.go:147)
• cmd/sin-code/internal/.sin-code/index.bin (via index_cmd.go:113)

This is dangerous for two reasons:

1. Source-tree leakage: a git status after running sin-code chat
   in a repo shows new lessons.db, sessions.db, or index.bin files
   in the working tree. The .gitignore only masks cmd/sin-code/tui/.sin-code/
   (issue chore: .gitignore — add cmd/sin-code/tui/.sin-code/ (runtime SQLite files) #61) — other workspace locations still leak.
2. Cross-project pollution: the workspace-relative files share one
   lessons.db per project, but the schema-wide defaults in
   internal/lessons/store.go:DefaultPath() and internal/session/store.go:DefaultPath()
   already point at ~/.local/share/sin-code/. The TUI is the
   one-and-only cmd/sin-code/... caller that bypasses the defaults.

The proper fix (cited verbatim by AGENTS.md §11.1) is to migrate the
TUI agent runner to the user-config-dir defaults, with workspace
isolation so lessons/sessions for project A and project B do not
collide.

Goals

1. agent_runner.go opens sessions.db and lessons.db from
   internal/session.DefaultPath() and internal/lessons.DefaultPath()
   by default — or, when Config.DBHome is set, from <DBHome>.
2. Per-workspace isolation: each workspace gets a sub-directory keyed
   by a stable short hash of cfg.Workspace, so two projects do not
   share lesson fingerprints.
3. The cmd/sin-code/internal/.sin-code/index.bin index migration is
   out of scope for this issue — the per-workspace index is a
   deliberate feature, not a leak (issue is tracked separately).
4. Add a one-shot migration message on first run: when an old
   project-relative .sin-code/ directory is detected, log a
   pointer to the new location and do not move the files (the
   user can run sin-code sessions migrate if desired — out of
   scope for v1 here).

Acceptance Criteria

• cmd/sin-code/internal/tui/agent_runner.go no longer writes
  <workspace>/.sin-code/{lessons,sessions}.db when
  Config.DBHome == "".
• New test: TestNewAgentRunner_UsesDBHomeWhenSet verifies the paths
  honour DBHome.
• New test: TestNewAgentRunner_DefaultsAreUserScoped swaps the
  package hook for lessons.DefaultPath / session.DefaultPath and
  asserts that the resulting files are NOT inside cfg.Workspace.
• AGENTS.md §11.1 updated: sessions.db and lessons.db rows are
  marked "moved to os.UserConfigDir() (issue [v3.8.0] feat: Vane bridge (citation-backed research) + stack command #62)"; the index.bin
  row keeps its current note.
• BACKLOG.md entry removed.
• CHANGELOG.md v3.22.0 entry added (or next release tag).

Non-Goals

• Auto-migration of existing per-workspace .sin-code/ directories.
• Re-hashing or re-fingerprinting existing lessons.
• The index.bin location.
• New sin-code sessions migrate subcommand.

References

• AGENTS.md §11.1 — the existing roadmap note.
• BACKLOG.md — the backlog entry.
• cmd/sin-code/internal/lessons/store.go:56-66 — DefaultPath().
• cmd/sin-code/internal/session/store.go:80-85 — DefaultPath().
• cmd/sin-code/internal/tui/agent_runner.go:147-161 — the bypass.