cli: drop `burn rebuild-index` (already an alias for `burn rebuild --index`)

[willwashburn]

Summary

burn rebuild-index is a thin alias for burn rebuild --index. The help text already documents it that way (packages/cli/src/cli.ts:44). Drop the dedicated subcommand and the alias entry — one verb is enough.

Background

runRebuildIndex is a 9-line wrapper around rebuildIndex() that reproduces what runRebuild already does when --index is set:

• Standalone command: packages/cli/src/commands/rebuild-index.ts:1-9
• Equivalent --index branch: packages/cli/src/commands/rebuild.ts:71-76
• Dispatch + help entries: packages/cli/src/cli.ts:17, cli.ts:44, cli.ts:127-128

There's no behavioral difference today. Carrying both verbs costs help-screen real estate and a second code path to keep in sync.

Implementation Steps

No users yet — this lands as a clean break with no deprecation alias path.

1. Delete packages/cli/src/commands/rebuild-index.ts.
2. Remove the runRebuildIndex import and the case 'rebuild-index' branch in packages/cli/src/cli.ts.
3. Remove the burn rebuild-index line from the help block in cli.ts.
4. Add a CHANGELOG entry under [Unreleased] noting the removal — users should run burn rebuild --index instead.
5. Sweep docs (README, AGENTS.md) for stray rebuild-index references.

Out of scope

Anything else about burn rebuild's shape — see the companion issue for unifying rebuild and archive.