fix(bin): resolve relative homes before persisting paths

[sparkus]

Intent

Prevent relative FM_HOME, FM_STATE_OVERRIDE, and FM_DATA_OVERRIDE values from baking unresolvable durable paths into generated charters or cross-process state, while preserving absolute path spellings unchanged.

What Changed

• Resolve relative FM_HOME, FM_STATE_OVERRIDE, and FM_DATA_OVERRIDE values before generating briefs, launching agents, starting AFK mode, or persisting X-mode poll shims.
• Preserve absolute path spellings, ignore inherited CDPATH, and report named errors when relative directories cannot be resolved.
• Document the durable-path behavior and add regression coverage for relative homes, overrides, launch routes, and secondmate status paths.

Risk Assessment

✅ Low: The change is well-bounded, and the corrective commit closes the prior CDPATH path-redirection defect at every affected normalization boundary with focused competing-directory coverage.

Testing

At exact target 2fb2366 over base 488cbf7, four focused path-consumer test scripts passed; a retained CLI transcript demonstrates relative-home charter generation, absolute parent-status routing, persisted correlated output, and fail-loud invalid input, with the worktree left clean.

Evidence: Relative secondmate CLI transcript

$ FM_HOME=home bin/fm-brief.sh routing-sm --secondmate --no-projects

$ generated charter status/reporting lines
27:Optional helper: `bin/fm-secondmate-report.sh` can append a correlated status line for you, but a plain `echo` that includes the same `corr=<id>` is equally valid - do not depend on the helper being present.
36:   `echo "{state}: {one short line}" >> '/private/var/folders/bd/ky17fvwx45353ky4s2bg8ys80000gn/T/no-mistakes-evidence/01KYQK920C83W99WPGD8SSZGDT/relative-secondmate-cli/home/state/routing-sm.status'`

$ secondmate appends a real completion event to the charter destination
completed: corr=demo-relative-path absolute route verified

$ unresolved relative FM_HOME fails loudly
exit=1
error: FM_HOME directory cannot be resolved: missing-home

Pipeline

Updates from git push no-mistakes

✅ **intent** - passed

✅ No issues found.

✅ **Rebase** - passed

✅ No issues found.

🔧 **Review** - 1 issue found → auto-fixed ✅

• 🚨 bin/fm-brief.sh:75 - Relative-path normalization inherits CDPATH. With an exported value such as CDPATH=.:.., cd "$path" can print its destination into the command substitution, producing a multi-line path; it can also select a same-named directory outside the caller's working directory. That corrupts the durable status/launch path this change is intended to fix. Clear CDPATH and use cd -- "$path" at this boundary and the equivalent sites in fm-spawn.sh, fm-afk-launch.sh, and fm-bootstrap.sh. Add a regression case with non-empty CDPATH.

🔧 Fix: Ignore inherited CDPATH during relative path normalization
✅ Re-checked - no issues remain.

✅ **Test** - passed

✅ No issues found.

• Inspected git diff 488cbf7804ab3f6b0625941df66f4df2fc869e02..2fb236610d879c34f1c66ecb36ac2615bf7834af and verified exact HEAD/base ancestry
• bash tests/fm-brief.test.sh
• bash tests/fm-spawn-dispatch-profile.test.sh
• bash tests/fm-afk-launch.test.sh
• bash tests/fm-x-mode.test.sh
• Generated a secondmate charter using relative FM_HOME=home with a conflicting CDPATH, verified its absolute state/routing-sm.status route, appended and read back a correlated completion event, then verified missing relative FM_HOME exits 1 with a named diagnostic
• Verified git status --short remained empty after testing

✅ **Document** - passed

✅ No issues found.

🔧 **Lint** - 1 issue found → auto-fixed ✅

• ⚠️ linter found issues (exit code 1)

🔧 Fix: Fix empty CDPATH assignments for ShellCheck
✅ Re-checked - no issues remain.

✅ **Push** - passed

✅ No issues found.