feat(brief): make ship-brief Project memory section opt-out via config knob

[aleb-moviestarplanet]

Make the ship-brief Project memory section opt-out

bin/fm-brief.sh unconditionally told every ship crewmate to commit knowledge into a project's AGENTS.md/CLAUDE.md, which conflicts with a captain preference that those files must never be committed into a project repo. This adds a local, gitignored config/project-memory knob so a fleet can opt out without changing the tracked default for every other firstmate user.

Changes

• Add bin/fm-project-memory-lib.sh: fm_project_memory_value/fm_project_memory_enabled read config/project-memory (absent or on = enabled, off = disabled, anything else fails open to on).
• bin/fm-brief.sh omits the whole "Project memory" section (no stub) from generated ship briefs when the knob is off; unchanged byte-for-byte when on/absent.
• bin/fm-ensure-agents-md.sh refuses loudly when the knob is off, so the opt-out is structural, not only a matter of the brief never mentioning it. It resolves its config directory from FM_HOME/FM_CONFIG_OVERRIDE when set, else from its own tracked-code-root location - this reliably covers an ordinary crewmate (which has no FM_HOME in its pane per bin/fm-spawn.sh) invoking the exact path baked into its brief, but not a secondmate home's own distinct value when that secondmate shares the primary's tracked checkout. Documented as a known limitation rather than silently assumed complete.
• Docs: AGENTS.md section 2 gets a one-line config entry, sections 6 and 11 get a one-line cross-reference to the new conditional behavior, and docs/configuration.md gets the full "Project memory" writeup.
• Tests: extended tests/fm-brief.test.sh and tests/fm-ensure-agents-md.test.sh (colocated, no new runner) to cover on/off/absent for both scripts.
• Unrelated fix found while validating: bin/fm-test-run.sh's --check-coverage guard compared LC_ALL=C-sorted files with plain comm, which spuriously fails under a non-C collation locale; pinned every comm call (and the matching one in tests/fm-test-run.test.sh) to LC_ALL=C to match the existing sort calls.

Deliberately not inherited into secondmate homes (like config/calm) and not added to FM_INHERITABLE_CONFIG, since this is a per-captain preference not requested for secondmate propagation and secondmates may legitimately work on projects with a different convention.