Add Thread Organizer example plugin

[brsbl]

What was wrong

The repository had no complete reference consumer for plugin-owned thread workflows, leaving native section icons/actions, runtime skill slots, protected unread routing, and durable sidebar ordering demonstrated only as disconnected primitives. The organizer skill also treated the remembered stage too much like a semantic classification even though new threads mechanically inherit the first configured non-Inbox stage until the user or bb organizer phase moves them. Finally, the settings description aligned its text box to the list panel's bounding edge, which left the visible capital R inside the rounded corner instead of on the corner's tangent.

What changed

• Add the Thread Organizer example with configurable workflow stages, protected Inbox behavior, remembered stage routing, native section icons/actions, CLI phase changes, and agent instructions.
• Persist and restore organizer-owned section identity, order, collapse state, and transitions while preserving unrelated sidebar positions.
• Fill the organizer skill's runtime workflow slot without mutating its bundled template.
• Teach the skill to re-evaluate at substantive task starts, resolved indirect kickoffs, scope changes, pre-implementation, implementation/validation transitions, failed validation, and handoff; update_plan does not move the bb workflow stage.
• Inset the settings description by the shared rounded-lg radius token (with the host's 0.5rem fallback), then hang Inter's measured capital-R sidebearing (180/2048em) so its visible ink edge lands on the panel corner's tangent without an arbitrary pixel nudge.
• Add the missing Turbo dependency from the example typecheck to @get-bb/plugin-sdk#build:types, so a clean checkout generates the bundled SDK declarations before checking the example.
• Keep PluginAgentConfiguration.skills as string[], keep experimental_skillSlots optional and experimental, and leave shared/default SDK artifacts plus PLUGIN_SDK_VERSION at 0.4.10. Only this example declares the planned engines.bbPluginSdk >=0.4.11 capability floor; no SDK 0.4.10 artifacts are vendored or relabeled.

Before — parent PR head be87f190

After — workflow feature evidence at 0d6967a96

Final optical-alignment detail — exact final head fe815c9dc

How you verified

• Node 22 with Turbo: 42/42 focused example tests pass across core guidance, server behavior, sidebar actions/controller, and app registration.
• Turbo typecheck passes for bb-plugin-thread-organizer-example and all upstream tasks (6/6 combined test/typecheck tasks).
• Built the exact final example head with the local source CLI: pnpm bb:dev plugin build examples/plugins/thread-organizer.
• Removed generated untracked examples/plugins/thread-organizer/types/ files after the build and confirmed a clean worktree.
• git diff --check be87f190be2ac7c69b9112048875f91600f83fc6..fe815c9dc36d5886faca106106dd5e2b0792e129.
• Desktop visual QA used the same isolated dev profile, fixture data, routes, interaction states, and 1224×768 native window bounds. Captures came only from desktop dev apps launched from the compared revisions.
• Exact final-head optical measurement: the list panel's left edge is 421px, its rounded-lg radius is 8px, and its corner tangent is 429px; the capital R's predicted visible ink edge is 429.001953125px, a 0.001953125px delta.
• Exact-head full CI run 32543758381: 9/13 jobs passed. The four failures reproduce the inherited baseline outside this stack's diffs: stale generated plugin-registry r/icon.json, the Machines settings 2 projects expectation, and Machine settings response-body reuse (reported in two app shards/check groups).

Fixes # — no linked issue.

BB-Thread-ID: thr_eibej5m9xw

AGENT GENERATED: by GPT-5.6-Sol