feat: add Canonical Thread Handoff protocol (Closes #505)

Define CTH comment types, base template, parser/validator helpers, protocol
documentation with examples, and workflow/runbook/prompt updates so agents
discover and post authoritative thread handoffs before acting.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

skills/llm-project-workflow/templates/canonical-thread-handoff.md

@@ -0,0 +1,36 @@
+# Template: Canonical Thread Handoff (CTH)
+
+Copy, fill the fields, and post as an issue or PR comment.
+
+```text
+## CTH: <Type>
+
+Status: <current workflow state>
+Next owner: <author|reviewer|merger|controller|operator>
+Current blocker: <none or exact blocker>
+Decision: <what was decided this session>
+Proof: <commands, SHAs, gate outputs, or explicit pending proof>
+Next action: <one concrete step for the next actor>
+Ready-to-paste prompt: <full prompt the next session should paste>
+```
+
+Allowed `<Type>` values:
+
+- State Handoff
+- Controller Decision
+- Author Handoff
+- Reviewer Handoff
+- Merger Handoff
+- Supersession Notice
+- Blocker
+
+Rules:
+
+- Find the latest CTH comment in the thread before starting work.
+- Treat the latest valid CTH as the current source of truth.
+- Post a new CTH when you finish, block, skip, supersede, approve, request
+  changes, or hand off.
+- A CTH summarizes workflow state; formal Gitea review verdicts remain
+  authoritative for merge gates.
+
+Full protocol: `docs/canonical-thread-handoff.md`