feat(bot): add owner-bounded application capabilities

[hmans]

Summary

• define a shared, server-owned application capability catalogue with human-readable disclosure metadata
• require every bot operation to intersect an explicit grant, the owner's live authority, and an explicit conversation context
• show approved capabilities and the bot's owner-written data-handling description on management and profile surfaces
• add the bot-only direct-message runtime API for listing/reading explicit bot DMs and posting text replies

This is PR 2 of the bot-account stack and depends on the bot identity/credential foundation.

Design

• ADR-071: Owner-bounded application capabilities
• FDR-038: Bot Accounts
• additive experimental public API; clients discover the exhaustive catalogue rather than hard-coding it

Verification

• mise test-cli
• mise test-frontend
• docs website build
• protobuf generation and compatibility check
• mise license-check

Stack

1. Bot account foundation
2. Owner-bounded application capabilities (this PR)
3. Contextual room and thread access