feat(desktop): start thread-bound remote sessions from mentions

[tee8z]

A sandbox agent must start in the thread where its owner mentioned it. The desktop starts agents without any thread information. This PR passes the published mention's channel and canonical thread root to the agent start, and native deploy binds them into the provider configuration.

sequenceDiagram
    participant C as useMentionSendFlow
    participant R as Relay
    participant S as useDetachedAgentStart
    participant N as deploy_to_provider (Tauri)
    participant P as Kubernetes provider
    C->>R: Publish the mention
    R-->>C: Published event (id, tags)
    C->>S: Start agent with publishedRemoteSession(event, channel)
    alt Sandbox agent and no thread scope
        S-->>C: Error toast, no start
    else Thread scope present or not a Sandbox agent
        S->>N: Start with sessionScope
        N->>N: Take the per-agent deploy lock and rebuild the payload
        N->>N: bind_sandbox_scope sets sandbox and identity_policy (sandbox: true only)
        N->>P: deploy
    end

Loading

• usesThreadSandbox selects agents with provider kubernetes and sandbox: true. A saved deployed status never skips the start for these agents.
• The thread root is the reply's root, or the message itself. It must be a lowercase 64-character event ID.
• In-flight start deduplication includes the thread scope, so two threads start two sessions.
• Provider config fields accept JSON objects and arrays, with at most 4 nesting levels, 64 array entries, and 20 fields per object. Secret-like keys are refused at every depth.

Stack

Position 6 of 20 in the DEV-2884 Buzz stack. Base: #12. Next: #13.

Validation

• pnpm typecheck, pnpm check (Biome, px-text, pubkey truncation), and pnpm check:file-sizes: pass. Biome reports the same 3 warnings as the base.
• pnpm test: 6,822 passed and 160 jsdom passed, 0 failed.
• cargo fmt --check in desktop/src-tauri: pass.
• This layer's tree is identical to the previous single-commit PR head (e4e9cd8b9).

Limits

• The Tauri Rust (provider_deploy.rs, backend.rs) is not compiler-checked locally. The glib-sys build needs GLib/GIO development libraries that this machine lacks.
• Buzz AGENTS.md requires a human to test the change before it leaves draft.

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