Tasks: declared skips in the process graph

Phase 4 of 4. Derived from the locked design.md and testing-plan.md — each task's _Test:_ names a row of the matrix. Ticket: issue #177.

Task list

• [x] T1 — model: the skip vocabulary — Node.skippable (+ as_mapping), Graph.skip_sets, expand_skip_tokens(), and the three compile validations (required×skippable, missing skipped edge, bad set member). Depends on: —. Requirements: R1.1–R1.4. Test: M1.
• [x] T2 — state: the declaration record — GraphState.skips loaded/saved additively; absent key → {}. Depends on: T1. Requirements: R2.1, R3.3. Test: M4, M12 (n/a-by-construction, exercised via absent-key loads).
• [x] T3 — the shipped outer loop declares — six skippable: true markers, six on: skipped edges, skipSets.spec-chain; pdlc-pr-loop untouched. Depends on: T1. Requirements: R1.5, R1.6, R4.1. Test: M2.
• [x] T4 — runtime: route, report, declare, snapshot — declared_skips() defensive filter; skip routing in start/advance (including the no-pointer advance path); provenance in status() both modes with forged-declaration warnings; module-level declare_skips() (reason required, entered/past refused, audit comment, event); label snapshot at entry via get-labels, best-effort, once. Depends on: T2, T3. Requirements: R2.1–R2.7, R3.1–R3.3, R3.5. Test: M3, M4, M6, M7.
• [x] T5 — artifact gates tolerate declared absences — HookContext.skipped_artifacts computed in evaluate(); validate-artifacts reports an absent, skip-covered slot as an info skip; present artifacts gated unchanged. Depends on: T4. Requirements: R3.4. Test: M5.
• [x] T6 — the operator surface — core.graphs.skip(), POST /graph/skip, the OpenAPI contract entry, the-loop graph skip subcommand, graph show printing skippable; no MCP exposure. Depends on: T4. Requirements: R2.3–R2.6, R4.2. Test: M7, M8.
• [x] T7 — docs and the paper trail — reference/workflow.md § Declared skips, SKILL.md rule bullet, docs/capabilities/process-graph.md behaviour + history row, docs/decisions/decision-067.md (+ index), labels report note. Depends on: T5, T6. Requirements: all (documentation of). Test: M9 (markdownlint; docs parity).
• [x] T8 — verification — execute the testing plan, fill § Verification results, commit evidence. Depends on: T7. Requirements: all. Test: M9 plus the plan itself.