feat(desktop): add agents without starting

[ismcaldwell]

Summary

Adds an explicit Add without starting action for stopped local managed agents in the channel members sidebar.

The new path:

• publishes channel membership without calling either managed-agent start command;
• verifies through the production agent-list seam that the agent remains stopped;
• reports unconfirmed membership and post-membership status drift as visible failures; and
• leaves the existing row-click and Add and start behavior unchanged.

The two actions have distinct accessible labels. The regression coverage exercises pointer and keyboard activation and asserts that no start command is emitted.

Related issue

Fixes #7202.

Related but distinct: #4835 concerns finding and re-adding removed personal agents, while #6468 concerns externally managed runtimes.

Testing

• pnpm check (existing unrelated warnings only)
• desktop unit suite: 5,886 passed
• focused Playwright: existing managed-agent invite/remove plus new stopped-agent no-start flow, 2 passed
• production desktop build
• workspace Rust formatting, clippy, and all ten infra-free Rust unit groups (including 899 buzz-acp tests)
• web production build
• Flutter analyze and mobile suite: 2,019 passed
• git diff --check

The aggregate local just ci command could not finish desktop-tauri-test because the Mac ran out of disk while linking the unchanged Rust desktop library. No Rust or Tauri source is modified by this PR; the repository-hosted CI remains the authoritative full-platform gate.

Before

After

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is 42b42447b0dc47c3e1a4d95caab893567dcd1677...588efb71409cf96a0492921686c63796b9199fa3.
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[ismcaldwell]

Before and after

Before — stopped local agents exposed a single Add action, which also started the runtime:

After — stopped local agents expose separate Add without starting and Add and start actions: