docs: LLM-Agent-SHA opaque attribution convention, Phase 0 (#86)

Implements the Phase 0 owner decision on #86 (issuecomment-1354):

- docs/llm-agent-sha.md: format llm-<12 lowercase hex> (^llm-[0-9a-f]{12}$),
  generation rules, per-PR/workstream lifetime, visible markdown metadata
  blocks, no SHA in branch/worktree names, same-SHA vs same-user vs
  same-profile distinction. Attribution only — never an eligibility input.
- docs/llm-workflow-runbooks.md: attribution subsection + handoff/review
  runbook pointers.
- templates start-issue.md / review-pr.md: handoff and review metadata
  blocks; reviewer rule that a different SHA is not a different actor.
- tests/test_llm_agent_sha.py: negative tests — same Gitea user with a
  different LLM-Agent-SHA still fails self-review and self-merge; eligibility
  results are identical with/without/across SHA env values; no gate accepts
  or reads any agent-SHA input.

No launcher/env handling, no gitea_whoami fields, no PR auto-injection, no
audit schema changes. No eligibility behavior changed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

skills/llm-project-workflow/templates/review-pr.md

@@ -8,6 +8,8 @@ Task: review PR #<pr> for issue #<n>.
 Rules (llm-project-workflow):
 - Review in a SEPARATE detached review worktree, never the author's folder.
 - You must NOT be the PR author. If the authenticated user == PR author, stop.
+  A different LLM-Agent-SHA does NOT make you a different actor — only a
+  different authenticated Gitea user does (docs/llm-agent-sha.md).
 - Do not merge if any check fails.
 
 Steps:
@@ -21,6 +23,14 @@ Steps:
 6. Run the test suite; note results.
 7. Post the review verdict: approve only if scope is clean and checks pass;
    otherwise request changes with specifics. Never merge from this review step.
+   Include a "Review Metadata" block (attribution only — docs/llm-agent-sha.md):
+
+   Review Metadata:
+   - LLM-Agent-SHA: llm-<12 lowercase hex, e.g. llm-41d0e7aa9f2c>
+   - LLM-Role: reviewer
+   - Authenticated-Gitea-User: <whoami result>
+   - MCP-Profile: <profile name>
+   - Eligibility: passed/failed
 
 Handoff: reviewer identity, PR author, scope verdict, checks + results, decision.
 ```