#30848 Bind remote Cloud repository frames to the surface chat

A linked Mac could send a Cloud repository action with any chat session, even when the frame named a different surface. Require remote list/add/attached/wait frames to name the session currently bound to their surface, using the same stored lookup as agent-session attach. Refused requests return remote_denied before repository work starts. Local CLI and agent behavior remain unchanged, and catalog sync remains unscoped.

Part of #30628. Supersedes #30830, whose branch name collided with existing backend work; this PR carries only the surface-binding change from the current base. The peer-picked Cloud route is a separate follow-up; it must resolve a real machine surface and its session before advertising Add repository. Existing Cloud-button clients already send both fields.

Tests cover matching frames, a different/missing session, missing/unknown/unbound/malformed surfaces, a surface rebound to a new session, sync, and revoked links. Validation before the clean cherry-pick onto current base: cmux-ci run 41303025723 passed at b0f0646928e02f5e26cfbfd9790364499af02434: cargo test -p cmux-tui-core --lib server::cloud_repos::tests (1 passed; core test binary compiled).

Agent session: agent session

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Summary by cubic

Linked Macs can no longer target a different chat when sending Cloud repository frames: remote list/add/attached/wait frames must name the session currently bound to their surface. Requests that don't match return remote_denied before repository work starts; cloud.repo.sync is exempt because sync has no chat. Local CLI and agent behavior are unchanged.

• Reuses the same surface-to-session lookup as agent-session attach by making tab_session shared.
• Tests cover matching and mismatched sessions, missing/unknown/unbound/malformed surfaces, surface rebinding, and revoked links.
• Part of #30628 and supersedes #30830; the peer-picked Cloud route is a separate follow-up.