Set GOTMPDIR per task so interrupted Go builds don't leak /tmp

[e-jung]

Problem

Go's GOTMPDIR is unset, so every go build / go test in a crewmate pane creates numbered temp dirs (/tmp/go-build*). Go cleans these on a clean exit but leaves them behind when a build is interrupted — signal, timeout, OOM, or full disk. Over time these accumulate and fill the disk on the firstmate VPS.

Fix

Give each task its own Go temp dir that teardown cleans deterministically.

• bin/fm-spawn.sh creates a per-task temp root /tmp/fm-<id>/ with Go's build temp nested at /tmp/fm-<id>/gotmp/ (Go won't mkdir GOTMPDIR, so it must exist first). It export GOTMPDIR=... into the crewmate's pane shell so the agent and every child process inherit it, and records tasktmp= in the task's meta.
• bin/fm-teardown.sh reads tasktmp= from meta and rm -rfs the whole root on cleanup, so interrupted builds can never outlive the task.

GOTMPDIR (not TMPDIR) is the targeted knob — TMPDIR is too broad (it redirects every program's temp, not just Go's). The nested root is extensible: other per-task temp can live under /tmp/fm-<id>/ later, and teardown cleans one deterministic path.

Backward compatibility

Tasks spawned before this change have no tasktmp= line in their meta. Teardown reads the value with an empty-tolerant grep and treats the empty string as a no-op ([ -n "$TASK_TMP" ] && rm -rf ...), so pre-existing tasks tear down exactly as before.

Safety net

The daily ~/.local/bin/fm-disk-cleanup.sh cron (rm -rf /tmp/go-build* ...) remains in place and sweeps any pre-fix stray dirs or anything missed in edge cases.

Tests

tests/fm-gotmp.test.sh (TAP style) covers:

1. fm-spawn creates gotmp/ and records tasktmp= in meta (structural contract + behavioral simulation).
2. fm-teardown removes the dir pointed to by tasktmp= (runs the real teardown via symlink against a fake root — exercises actual code, sources nothing).
3. fm-teardown skips gracefully when tasktmp= is absent (backward compat).
4. fm-teardown skips gracefully when tasktmp= points to a nonexistent dir.

All existing tests (fm-spawn-batch, fm-wake-queue) still pass.

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