Bump keep to v0.4.6: multi-group co-signer + active-group switcher

[kwsantiago]

Bumps the bundled keep co-signer to v0.4.6 and registers it in the StartOS version graph.

What's new since v0.4.3

• Multiple key groups (v0.4.6): the co-signer no longer crashes when the vault holds more than one FROST group. It auto-selects a group to serve and you can switch which group is served from the Web Admin's Shares section (single-flight, no-op on re-select).
• Default relay → wss://bucket.coracle.social (reliable FROST coordination), pre-populated in the UI.
• Co-signer reliability: accurate online/offline presence + stale-nonce fallback (v0.4.5).
• Restored the mobile pre_approve binding (v0.4.4).

Package changes

• Submodule keep pinned to v0.4.6 (61cdae8).
• Added startos/versions/v0.4.6_0.ts (no-op migration), set current in versions/index.ts.
• instructions.md: clarified the optional Group (npub) override + the Shares switcher.

Typecheck (tsc --noEmit) and s9pk pack verified locally.