Add portability-checker agent

Read-only subagent that audits a repo against the vendor-neutral /
hot-swap standard: AGENTS.md canonical with a CLAUDE.md symlink, scoped
guides symlinked from .claude/rules with index lines, thin wrappers, and
relative correctly-directed symlinks. Substance in guides/, thin Claude
wrapper in adapters/, catalogued in the subagents handbook.

subagents-handbook.md

@@ -84,6 +84,7 @@ least one.
 | reviewer | independence | Fresh-eyes review of a diff before commit | ✅ in kit |
 | security-auditor | independence | Hostile persona: vulns, secrets, dependency CVEs | ✅ in kit |
 | spec-checker | independence | Compliance vs Start9 community registry requirements | ✅ in kit (`start9-spec-checker`) |
+| portability-checker | independence | Compliance vs my vendor-neutral / hot-swap standard | ✅ in kit |
 | exerciser | containment | Black-box QA: run it, feed it normal + hostile inputs | ✅ in kit |
 | researcher | compression | Multi-source web research → cited brief | ✅ in kit |
 | test-runner | compression | Run the suite, return only failures + causes | build when a repo has a real suite |
@@ -130,9 +131,9 @@ the catalog. The security-auditor runs them.
    capability ceiling. Both live in the wrapper frontmatter; frontmatter effort
    overrides the session level while that agent runs. Match the dial to where the cost
    actually lives: evaluator and security-auditor are **opus/high** because the report
-   *is* the reasoning; reviewer, researcher, exerciser, and spec-checker are
-   **sonnet/medium** because their cost is dominated by frequency, tool-call turns, or
-   procedure-following — not thinking depth. The deep insight: a well-proceduralized
+   *is* the reasoning; reviewer, researcher, exerciser, spec-checker, and
+   portability-checker are **sonnet/medium** because their cost is dominated by frequency,
+   tool-call turns, or procedure-following — not thinking depth. The deep insight: a well-proceduralized
    guide is *pre-computed reasoning*, so every hour spent tightening a guide lowers
    the model+effort that task needs forever. `low` is for truly mechanical agents
    (migrator, test-runner) once their guides are trusted; `xhigh` is the upgrade if an
@@ -192,8 +193,8 @@ softened.
 
 ### Length budgets
 
-Reviewer ≤ 70 lines · exerciser & spec-checker ≤ 80 · researcher & security-auditor
-≤ 100 · evaluator ≤ 120. Tighten freely; loosen only with cause.
+Reviewer ≤ 70 lines · exerciser, spec-checker & portability-checker ≤ 80 · researcher &
+security-auditor ≤ 100 · evaluator ≤ 120. Tighten freely; loosen only with cause.
 
 ---
 
@@ -279,7 +280,9 @@ Per repo, in order:
 1. **Retrofit first** (per `retrofit-playbook.md`): AGENTS.md with Current state, the
    CLAUDE.md symlink. Subagents load this file — the evaluator literally reads it to
    judge the build against your stated intent. Eval before retrofit = grading against
-   a blank rubric.
+   a blank rubric. Confirm the retrofit's structure with **portability-checker** before
+   moving on — especially on repos retrofitted under an older playbook, where AGENTS.md
+   may still be a real CLAUDE.md or rules may not yet be symlinks into docs/guides.
 2. **`/full-eval`** in a fresh session. Walk away; read one synthesized report.
 3. **Triage** into the repo's AGENTS.md Current state: P0/P1 become the work queue,
    P2 becomes a "known debt" list, P3+ gets deleted or done in bulk.
@@ -296,6 +299,8 @@ Per repo, in order:
 - **Every meaningful diff:** reviewer, before commit. This is the habit that compounds.
 - **Before any release:** exerciser + security-auditor in parallel.
 - **Before registry submission:** start9-spec-checker.
+- **After retrofitting or converting a repo:** portability-checker, to confirm it follows
+  the vendor-neutral / hot-swap standard before you rely on it being swappable.
 - **Quarterly-ish:** `/full-eval` on actively maintained repos; drift accumulates
   silently.