Make composer selection reactive and move Automation/Plugin rows into plugins

[ymichael]

Human comments

What was wrong

After #4474, plugins could write composer picker settings but could not reactively read them. The Automation and Plugin creation rows were also hard-coded in the core + menu, so the plugins that own those features could not contribute them. The menu's command rows edited the draft relative to the cursor (replacing a preceding action or a half-typed command, skipping duplicates), which plugins cannot reproduce because they cannot see the cursor.

What changed

• Added composer.selection: ComposerSelection | null, with stable snapshots and subscriptions in useComposer and useComposers. It reports the thread (including handoff) and new-thread pickers, and the read-only pickers of queued-message and sent-message editors: the thread's provider with the queued message's or the thread composer's settings. Composers without pickers return null. setSelection retains partial updates.
• Plan, Goal, Automation, and Plugin rows now put their command or prompt at the start of the draft, replacing a leading command pill; clicking the same row again changes nothing. Commands work anywhere in a message, but Automation and Plugin are sentence starters and /goal is passed to Codex as text, so the start is the only position that matters. This removes the cursor-based replacement and duplicate logic from [codex] Replace prompt plus menu PR #194; a half-typed /pl is no longer replaced. Skills still inserts its trigger at the cursor because it opens the skills picker.
• Moved the Automation row into the Automations plugin and Plugin creation into default-enabled BB Guide. Both insert their sentence ("Create a new bb automation to" / "Create a new bb plugin that") with the existing composer.replace; there is no new composer method. Core applies the same rule to its provider-declared Plan/Goal rows. Explicit menu-row icons now take precedence over plugin branding, so both rows keep their original icons.
• Both rows appear in the same composers as on main, including the sent-message editor: plugin + menu rows are exempt from its customization suppression, while actions, banners, send-menu rows, and rich text remain suppressed there.
• Removed the /automation macro from core: it is no longer offered in slash autocomplete, converted on paste, or expanded to the automation sentence on send (along with the legacy /loop alias). Core no longer knows about Automations. An unsent local draft that already holds an /automation pill now sends it as literal text.
• Bumped the SDK to 0.6.2 for selection. Automations and BB Guide require SDK 0.6.0 for replace. Updated the Plugin Guide, authoring references, and API audit. These are client-local changes; no database, server/daemon wire, or CLI changes.

How you verified

• pnpm exec turbo run test --filter=@bb/app on the promptbox, plugin, embedded-chat, thread-detail, and lib suites plus useCommandSuggestions: 1931 passed. Covers the prefix rule (existing text, later pills shifting, replacing a leading command, repeated clicks), core/plugin row order and icons in new-thread, thread, and queued-message composers, menu rows re-rendering on selection changes, queued and sent-message selections (including a bottom handoff), and plus-menu rows under suppression.
• Turbo tests for client-core (277), the SDK (361), Plugin Guide (82), Automations (135), and BB Guide (7) passed, including new unit tests for the two plugins' draft transforms.
• Turbo lint/typecheck for the app, client-core, SDK, Automations, BB Guide, and Plugin Guide passed with no errors. The SDK version check passes.
• Embedded chat's queued host has no test harness and is covered by typecheck only. This version has not yet been checked in the app.

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