feat(bot): add contextual room access

[hmans]

Summary

• let room managers explicitly install/remove bots, including in Universal rooms, without granting channel read access
• turn a human-authored direct bot mention in a root or reply into access to that root's thread only; bot-authored and role/broadcast mentions do not invite bots
• let the inviter, bot owner, or room manager revoke a thread invitation, and clear all invitations on uninstall
• add bot-runtime thread list/read/write operations with capability, owner-membership, and owner-RBAC checks
• add a Bearer-authenticated, write-only incoming webhook for root text messages in explicitly installed channels

This is PR 3 of the bot-account stack and depends on the application-capability slice.

Security and delivery semantics

• bots still cannot enumerate/self-join rooms or use the ordinary API, realtime, or asset surfaces
• thread reads expose only the invited root and replies; uninstall/reinstall does not revive old grants
• privacy-bearing reads retain a durable authorization boundary through response assembly; room-manager revocations commit through the authorization fence
• webhook writes are re-authorised at commit and expose no read surface
• webhook delivery is at least once; an uncertain retry can create a duplicate (idempotency and rate limiting are follow-ups)

Design

• ADR-071: Owner-bounded application capabilities
• FDR-038: Bot Accounts
• FDR-019: Room Lifecycle
• additive experimental public API and additive persisted events/fields; buf breaking passes against origin/main

Verification

• mise test-cli
• mise test-frontend
• targeted webhook, bot-thread, projection, snapshot, and Universal-room UI tests
• Svelte analysis and changed-file ESLint
• docs website build
• protobuf generation and compatibility check
• mise license-check

Stack

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