feat: expose labels and label-scoped chat search

[b-pm]

Summary

Expose Beeper's existing read-only label API in the CLI and make labels useful for agent/operator workflows.

This adds:

• beeper labels list [--ids]
• label selector resolution by ID, exact name, or unique partial name
• beeper chats search --label <name-or-id>
• beeper chats search --label Work --unread for scoped unread queues
• optional chat-search query when a label/unread filter is supplied
• docs, generated command reference, manifest examples, and tests

Why

The Desktop API already exposes GET /v1/labels and labelID on chat search, but the CLI currently says labels are unsupported and offers no way to use them.

That blocks useful agent workflows such as:

• "what is unread in Work?"
• "show customer chats matching invoice"
• "give me the Community queue without scanning every chat"

Related: beeper/desktop-api-mcp#5.

Safety / scope

• Read-only only. The Desktop API does not currently expose label creation, assignment, rename, or removal, so this PR does not invent write semantics.
• Existing beeper chats search <query> behavior remains valid.
• A query may be omitted only when at least --label or --unread is supplied.
• Label names resolve locally to the API's stable label ID before chat search.

Tests

Adds:

• unit coverage for label list/resolution, including ambiguity and not-found behavior
• command-surface smoke coverage for labels list, --label, and --unread
• CLI-level integration coverage proving a label name is resolved via /v1/labels and sent to /v1/chats/search as labelID with unreadOnly=true

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