Refactor SDK-first architecture seams (#3316)

* refactor: tighten sdk-first architecture seams

Refs #3312

* refactor: finish state document seam cleanup

Refs #3312

* test: harden minimal install cleanup assertion

* ci: support sdk-scoped package lock

* fix(3316): restore root package-lock.json and align changeset pr ref

Reverts dec57a83 ("ci: support sdk-scoped package lock") and restores
the root package-lock.json that c249d34d deleted. The deletion was the
wrong direction:

- The root package.json declares its own runtime and dev deps
  (@anthropic-ai/claude-agent-sdk, ws, c8). Without a root lockfile,
  `npm install --no-package-lock` resolves whatever satisfies semver at
  install time — CI today and CI in six months can install different
  transitive trees, defeating reproducibility.
- The lockfile has been part of every release on this repo (long
  history on main); removing it loses the npm audit / Dependabot
  target without compensating benefit.
- The CI workaround pattern (cache-dependency-path: sdk/package-lock.json
  + `npm install --no-package-lock`) papered over the symptom rather
  than fix the cause.

Also fix the changeset pr: from 3312 (issue) to 3316 (PR). CONTEXT.md
flags this exact failure mode as a recurring CodeRabbit finding.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix: address coderabbit review findings

* fix: close remaining coderabbit threads

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sdk/src/query/phase-lifecycle.ts

@@ -30,7 +30,6 @@ import {
   phaseTokenMatches,
   toPosixPath,
   planningPaths,
-  stateExtractField,
 } from './helpers.js';
 import { extractFrontmatter } from './frontmatter.js';
 import { extractCurrentMilestone } from './roadmap.js';
@@ -41,6 +40,7 @@ import {
   releaseStateLock,
   stateReplaceField,
 } from './state-mutation.js';
+import { stateExtractField, stateReplaceFieldWithFallback } from './state-document.js';
 import type { QueryHandler } from './utils.js';
 import {
   assertNoNullBytes,
@@ -959,29 +959,6 @@ export const phaseRemove: QueryHandler = async (args, projectDir, workstream) =>
   };
 };
 
-// ─── stateReplaceFieldWithFallback (inline) ────────────────────────────────
-
-/**
- * Replace a field with fallback field name support.
- *
- * Tries primary first, then fallback. Returns content unchanged if neither matches.
- * Reimplemented here because state-mutation.ts keeps it module-private.
- */
-function stateReplaceFieldWithFallback(
-  content: string,
-  primary: string,
-  fallback: string | null,
-  value: string,
-): string {
-  let result = stateReplaceField(content, primary, value);
-  if (result) return result;
-  if (fallback) {
-    result = stateReplaceField(content, fallback, value);
-    if (result) return result;
-  }
-  return content;
-}
-
 // ─── updatePerformanceMetricsSection ───────────────────────────────────────
 
 /**