chore(rbac): remove obsolete Mongo index initializer

[kevkantes]

NEEDS DISCUSSION

@sriaradhyula — this removes the standalone RBAC Mongo index initializer introduced in #1407. Please confirm that no external/operator runbook still relies on it.

Related to #2444; this PR does not close that issue.

Description

Removes scripts/init-rbac-mongo-indexes.ts, an uncalled one-off initializer, and updates the canonical RBAC file map to point to the supported versioned migration registry instead.

Audit evidence:

• No tracked package script, Make target, workflow, deployment configuration, or current operator runbook executes it.
• The code-backed rbac_indexes_v1 migration in ui/src/lib/rbac/migrations/registry.ts owns RBAC index creation through the supported admin migration surface.
• The startup Mongo index path in ui/src/lib/mongodb.ts owns application-managed indexes.
• Historical Spec Kit references are removed by draft chore(repo): retire Spec Kit workflow and historical specs #2454; no current documentation points at the deleted script after this PR.

Type of Change

• Bugfix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (retire stale one-off script)

Pre-release Helm Charts (Optional)

Not applicable.

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests (not applicable; deletion)
• All relevant checks pass

Validation

• git diff --check main...HEAD
• Exact human-authored diff: delete scripts/init-rbac-mongo-indexes.ts; replace its canonical file-map entry with ui/src/lib/rbac/migrations/registry.ts.
• No runtime tests apply to a removed, uncalled script; the supported migration implementation is unchanged.

[github-actions]

✅ No proprietary content detected. This PR is clear for review!