Add scripts/install.sh: reproducibly generate the /faber command with a repo-derived path

[yihanzhu]

Goal / problem

The /faber slash command at ~/.claude/commands/faber.md was created by hand and hardcodes the control-plane repo path (~/git/fabrica/...) in ~6 places. It is not reproducible from this repo and not covered by RESTORE.md — which breaks both team goals: reusable-by-anyone (a new adopter can't regenerate it) and full-backup (it can't be reconstructed if lost). Today there is no scripts/install.sh at all.

Build an idempotent scripts/install.sh that generates ~/.claude/commands/faber.md with the control-plane repo path derived from the repo's own location (not hardcoded ~/git/fabrica), then prints next steps. This formalizes the hand-made command and makes the install reproducible.

Acceptance criteria

1. New scripts/install.sh — #!/usr/bin/env bash, set -euo pipefail, shellcheck-clean, executable.
2. Path derived, never hardcoded. Resolve the repo root from the script's own location (e.g. cd "$(dirname "$0")/.." && pwd, resolving symlinks) and substitute that absolute path everywhere the /faber command references the control plane. No ~/git/fabrica literal in the generated output.
3. Generates ~/.claude/commands/faber.md containing the current /faber behavior (read role files → operate on the current repo → issue/round/reviewer/merge-gate loop → never write code, never merge). Source the body from a template in this repo (e.g. templates/faber-command.md with a path placeholder) so the command text lives in version control and isn't duplicated inside the script. Creating that template is part of this issue.
4. Idempotent. Re-running produces the same result and overwrites cleanly; create ~/.claude/commands/ if missing. If an existing faber.md differs, back it up (e.g. faber.md.bak) before overwriting rather than clobbering silently.
5. Prints next steps on success: where the command was written, that the repo path was derived to <resolved path>, and the follow-on setup (scripts/setup-target-repo.sh in a target repo; confirm CI + optional CLAUDE.md). Match the output style of scripts/setup-target-repo.sh.
6. RESTORE.md — replace any "recreate the /faber command by hand" step with "run scripts/install.sh".
7. README.md — add scripts/install.sh (and the new template) to the Layout block with a one-line description.

Likely files

• scripts/install.sh (new)
• templates/faber-command.md (new — the parameterized command body)
• RESTORE.md (install step)
• README.md (Layout)

Test expectations

• CI green: install.sh shellcheck-clean, structure check intact.
• Verify (in PR description) a real run generated ~/.claude/commands/faber.md with the derived path and that a second run is idempotent.
• CI does not write to ~/.claude — static check only.

Out of scope

• Installing routines, Codex, or any connector setup (this is just the /faber command + its derived path).
• Changing the behavior/wording of the Faber role beyond parameterizing the path.
• A full multi-component installer — one concern: reproducibly generate the /faber command.