test: add spawn-guard placeholder regression test for fm-brief.sh

[prajwal-395]

What

Regression test that stops bin/fm-brief.sh from ever again emitting a scaffold token that its own sibling guard in bin/fm-spawn.sh then rejects.

Why

PR 5 added a spawn guard that refuses to launch when any {[A-Z_]+} token remains in the brief file. PR 8 (commit a9c8212) then fixed a case where the Herdr NOT-ENABLED section contained a literal {TASK} in its prose - "the task text that replaces {TASK} later" - creating a second occurrence that survived firstmate's placeholder fill and blocked dispatch for every non---herdr-lab brief.

Nothing prevented recurrence. This test closes that gap.

How

Extends tests/fm-brief.test.sh with test_scaffold_tokens_match_spawn_guard_expectations, which:

1. Reads the guard's own regex from bin/fm-spawn.sh (\{[A-Z_]+\}) rather than restating it, so the test cannot drift from the pattern it protects.
2. Generates every scaffold variant the script can produce:
   • Ship: no-mistakes, direct-PR, local-only
   • Scout
   • Secondmate charter: project-list and --no-projects
   • Each with and without --herdr-lab where that flag applies
3. Asserts exact per-token occurrence counts in the generated brief file - not just the unique set. An extra occurrence of an expected token (the exact original bug class) is caught, along with any novel token.

Defect-reintroduced run (test MUST fail)

Reintroduced the defect: put {TASK} back into the Herdr NOT-ENABLED prose.

not ok - ship no-mistakes no-herdr: {TASK} appears 2 time(s), expected 1

Defect-reverted run (test MUST pass)

ok - fm-brief.sh: no scaffold variant emits uppercase brace tokens beyond its expected placeholders (spawn-guard regression)
FM_TEST_SUMMARY total=1 failed=0 skipped_gate=0

Constraints satisfied

• bin/fm-spawn.sh is unchanged
• bin/fm-brief.sh is unchanged
• shellcheck is clean on the test file
• The guard regex is read from source, not restated