fix: GNU-first stat in file_mtime() to unbreak GNU/Linux CI (#245)

[kiki830621]

Problem

CI is red on main: the idd-tree-lock fixture suite fails on the GNU/Linux
runner (green on macOS) —

✗ f8 fresh unreadable lock -> held (not stolen): expected exit 3 got 1

A red main obscures the CI signal for every future PR, so this is the
highest-priority fix in the backlog.

Root cause — BSD-first stat breaks on GNU/Linux

scripts/idd-tree-lock.sh file_mtime() tried BSD syntax first:

file_mtime() { stat -f %m "$1" 2>/dev/null || stat -c %Y "$1" 2>/dev/null; }

The two stat dialects read the flags incompatibly:

flag	BSD / macOS	GNU / coreutils (Linux CI)
-f	format string (%m = mtime)	--file-system (prints multi-line fs status)
-c	illegal option → clean usage error, no stdout	format string (%Y = mtime)

On Linux, stat -f %m "$LOCK" is parsed as --file-system and prints the
lock's multi-line filesystem block to stdout. file_mtime() therefore returns
"<multi-line fs garbage>\n<real mtime>", which corrupts holder_is_live()'s
freshness arithmetic (age=$(( now - mtime )) on a multi-line string fails →
holder judged "not live" → a fresh empty lock is wrongly reclaimed instead
of held). Hence expected exit 3 got 1.

Fix

Try GNU-first, which resolves cleanly on both platforms:

file_mtime() { stat -c %Y "$1" 2>/dev/null || stat -f %m "$1" 2>/dev/null; }

• GNU/Linux: stat -c %Y succeeds → bare integer, || short-circuits (no fs garbage ever produced).
• BSD/macOS: stat -c %Y fails cleanly (illegal option, no stdout — verified rc=1, empty stdout) → || falls through to stat -f %m → correct mtime.

Test — RED/GREEN regression

New fixture f11 PATH-shims a GNU-semantics stat so the Linux-only failure
reproduces on any host: RED with the BSD-first order (reproduces the exact
expected exit 3 got 1 symptom on macOS), GREEN after the swap.

• Before fix: idd-tree-lock 20 PASS / 1 FAIL (f11).
• After fix: idd-tree-lock 21 PASS / 0 FAIL.
• Full CI entrypoint (run-all-tests.sh): 21 suites, 0 failed.

Verification

• bash plugins/issue-driven-dev/scripts/run-all-tests.sh → 21 suites, 0 failed (local macOS)
• CI Linux runner green on this branch ← the cross-platform check I can't run locally

Refs #245