Keep bulk messages from waking Completed threads

[ariofrio]

Summary

• Teach the Thread Stages skill to inspect workflow stages before bulk messaging or notifying staged threads.
• Reserve bb thread tell for intentionally resuming Completed roots and direct passive messages to bb thread queue create.
• Add a Thread Stages patch changeset.

Rationale

Thread Stages keeps Deferred, Blocked, and Completed placements fixed during automatic activity changes, including after the Ribbon sidebar migration in #49. That protection does not stop an explicit message from starting a provider turn, so bulk notifications can wake Completed work without moving its stage.

Validation

• uv run --with pyyaml python .../quick_validate.py plugins/bb-plugin-thread-stages/skills/thread-stages
• npm run check:layout
• env -u BB_CLI npm run build --workspace=bb-plugin-thread-stages
• npm run verify:package --workspace=bb-plugin-thread-stages
• npm exec --yes --package=@changesets/cli@3.0.0 -- changeset status --since origin/main (patch: bb-plugin-thread-stages)

Live reload was skipped because the installed Thread Stages plugin is sourced from another worktree, which the repository installer intentionally does not replace.

_{Written by Codex}