cleanup: retire the legacy /faber command bridge

[yihanzhu]

Retire only the legacy command bridge

This completes the repository half of the accepted rename Ops 4 command-entry cleanup. It does not retire .fabrica/** or FABRICA_* target compatibility and does not claim the full external Ops 4 checklist is complete.

What changed

• scripts/install.sh creates and updates only /yshifu.
• If the retired /faber filesystem entry still exists, including as a dangling symlink, installer warns loudly but never reads, backs up, overwrites, or deletes it.
• scripts/doctor.sh requires /yshifu; it no longer accepts the old command as fallback and fails while the retired entry remains.
• QUICKSTART documents the warning and points to the explicit cleanup.
• RESTORE gives a fail-closed operator sequence: verify current /yshifu, inspect the old file, move it outside command discovery to a unique owner-controlled backup, run doctor/full smoke, and use no-clobber rollback.
• The existing rename test now exercises fresh install, custom-file preservation, no implicit backup, dangling symlinks, installer warning, and runtime doctor failure.

Safety and scope

• This PR never deletes or mutates the live ~/.claude/commands/faber.md and never changes ~/.claude/settings.json.
• Existing custom content is left byte-identical.
• The post-merge live move is a separate operator action with a unique backup and rollback.
• .fabrica, legacy environment keys/markers, old domain redirect, settings cleanup, and historical artifacts are outside scope.
• No agent merges this PR.

Proof

Verified in a clean detached worktree at 32b553a285de48cb1579f19720a5076c39bb3ab9:

• git diff --check: passed.
• Independent builder, Security/Compliance, and cross-file consistency reviews: clean after dangling-symlink, installer-warning, doctor-runtime, backup-root, permission-bit, and no-clobber issues were fixed.
• ShellCheck 0.11.0 over tracked scripts: passed.
• Focused rename/retirement suite: 27 passed, 0 failed; pathname/probe guards passed.
• Required-file structure check: structure ok.
• North-star resolver: 74 passed, 0 failed.
• North-star gate/consumer: 226 passed, 0 failed.
• models.conf parser: 63 passed, 0 failed.
• degraded Codex gate: 91 passed, 0 failed.
• Remaining v2 suites and rename gate: passed.

Closes #167

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View logs

[yihanzhu]

Codex reviewer (cross-vendor, read-only)

Reviewed-head: 32b553a
Reviewed-base: d3284c3
reviewer: operator-default @ high

Posted verbatim by codex-review.sh (codex exec review --json --base refs/codex-review/168-9975/base in an isolated temp worktree, sandbox forced read-only). Comments only — Codex never pushes, approves, or merges.

The installer, doctor, documentation, and regression tests consistently retire the legacy command while preserving existing files and symlinks. No blocking functional regressions were found.