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>

docs/llm-workflow-runbooks.md

@@ -45,6 +45,18 @@ Use any eligible reviewer profile to review PR #N.
 Use any eligible merger profile to merge PR #N if checks pass.
 ```
 
+### Attribution: `LLM-Agent-SHA` (metadata only)
+
+Sessions may attribute their work with an opaque `LLM-Agent-SHA`
+(`llm-<12 lowercase hex>`, e.g. `llm-8f3a9c2d6b41`) in PR-body and
+review-handoff metadata blocks — see
+[`llm-agent-sha.md`](llm-agent-sha.md) for the full convention. It is
+**attribution only**: eligibility is decided solely by the authenticated
+Gitea user and the profile's allowed operations. Two sessions with different
+SHAs under the same Gitea user are the same actor — a different SHA never
+permits self-review or self-merge. Keep the SHA out of branch and worktree
+names.
+
 ## Prerequisites: canonical config + thin launchers
 
 Runtime profiles live in **one canonical JSON file**, referenced by every LLM
@@ -274,7 +286,8 @@ touching anything.
   `fix/...` / `docs/...`); `cd` into that worktree; implement narrowly; add or
   update tests if behavior changes; run the full suite; commit with an
   issue-linked message; open a PR to `master`. **Do not** review or merge your
-  own PR.
+  own PR. Include an `LLM Handoff Metadata` block (with `LLM-Agent-SHA`) in
+  the PR body — see [`llm-agent-sha.md`](llm-agent-sha.md).
 - **Prompt:** `Use an author profile to implement issue #N and open a PR to
   master. Do not self-review or self-merge.`
 
@@ -285,7 +298,11 @@ touching anything.
 - **Steps:** confirm identity + eligibility (menu eligibility check or
   `gitea_check_pr_eligibility`); read the diff; confirm scope matches the linked
   issue; post the review (`comment` / `request_changes` / `approve`) via the
-  gated review tool. Pin the reviewed head SHA where supported.
+  gated review tool. Pin the reviewed head SHA where supported. Include a
+  `Review Metadata` block (with your own `LLM-Agent-SHA`) in the review —
+  and remember: a different `LLM-Agent-SHA` does **not** make you a different
+  actor; only a different authenticated Gitea user does
+  ([`llm-agent-sha.md`](llm-agent-sha.md)).
 - **Prompt:** `Use any eligible reviewer profile to review PR #N. Approve only
   if scope matches issue #M and checks pass; otherwise request changes.`
 
@@ -391,6 +408,7 @@ scripts/release-tag v0.4.0 --notes-file /tmp/release-notes.md --push
 
 - [`../skills/llm-project-workflow/SKILL.md`](../skills/llm-project-workflow/SKILL.md) — portable cross-project LLM workflow skill.
 - [`gitea-execution-profiles.md`](gitea-execution-profiles.md) — the profile model.
+- [`llm-agent-sha.md`](llm-agent-sha.md) — opaque agent attribution metadata (never an eligibility input).
 - [`safety-model.md`](safety-model.md) — trust boundaries and audit logging.
 - [`tool-boundaries.md`](tool-boundaries.md) — per-tool allowed operations.
 - [`credential-isolation.md`](credential-isolation.md) — credential handling.