Tasks: publish a work item's spec chain as one Claude artifact

Task list

[x] A1: red tests. test_claude_artifact.py (gate, state, runtime, prompt) and the Slack pin, failing before the change. Req: R1–R4.2 · Test: T1–T6.
[x] A2: the gate. Token, rendering, parsing, harness resolution, confirmation, result data, frozen graph. Req: R1.1–R1.4, R2 · Test: T1, T2. Deps: A1.
[x] A3: the frozen record. WorkItemState.claude_artifact, the runtime freeze, the attribute catalogue, the archive and graph output. Req: R3 · Test: T3, T4. Deps: A2.
[x] A4: the prompt. GraphContext.claude_artifact and the outer-loop line. Req: R4.1, R4.2 · Test: T5. Deps: A3.
[x] A5: the Slack mirror. Non-phase token. Req: R1.5 · Test: T6. Deps: A2.
[x] B1: the skill. SKILL.md bullet, reference/workflow.md section, reference/collaboration.md surface row. Req: R4.3–R4.5. Deps: A4.
[x] B2: the docs. Capability docs (process-graph.md, spec-workflow.md) with history rows, docs/cli/commands/graph.md checklist. Deps: B1.
[x] C1: verify. Run the testing plan; record evidence/verification.md. Test: T7. Deps: A1–A5.
[x] C2: review and ship. Self-review rounds, security review, documentation record, reviewer briefing, PR. Test: T8. Deps: B2, C1.