Fix INDEX.md staleness across all TODO-mutating paths. 1.19.0 wired the
`todo reindex` helper into /sf:todo and /sf:done; this release closes the
remaining gaps in /sf:plan (rm), /sf:triage (create), /sf:revise (deferred
TODOs), /sf:priority (priority edit), /sf:migrate-todos, and the
sf-spec-reviser agent. /sf:todos now delegates INDEX layout to the same
helper instead of writing it inline — the reindex routine is the single
source of truth.

Safety net: new `todo check-stale` CLI subcommand reports drift between
TODO-*.md and INDEX.md; /sf:status invokes it and surfaces an "INDEX.md
stale" warning with the specific divergent IDs.

Header text in INDEX.md rewritten to describe actual behaviour (cache
refreshed by /sf:todos OR by the helper) instead of the old misleading
"auto-generated" framing.

9 new tests in tests/todo-index.test.cjs covering reindex idempotency,
header content, drop-after-delete, and all check-stale scenarios.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>