Move the code and terraform audits into the reviews plugin

Copy the standalone code-review and terraform-review skills into
plugins/reviews as audit-code and audit-terraform. The rename separates the
automated, linter-driven audits from the guided review-pr walkthrough that
already lived here.

Resolve bundled script paths through ${SKILL_DIR}, exported in a new step 0.
CLAUDE_PLUGIN_ROOT is not set in the Bash tool environment, so the obvious
substitution would have expanded to nothing and broken every collection
script invocation.

Replace the PLAN and DESIGN docs with READMEs written from the current
SKILL.md and scripts. The old docs had drifted badly: they named semgrep
where the code calls opengrep, scoped five review agents where there are
now eight, and predated Lua, PowerShell, and GitHub Actions support.

Add CONSISTENCY_NORMS to the audit-terraform agent inputs. The collection
script writes consistency_norms.json and the agent prompt declares it, but
SKILL.md never listed it, leaving the variable unsubstituted.

Drop the --ingest-verdicts instruction from both skills. review_stats.py
parses no arguments, so the ref-mode verdict template it told users to feed
back could never be read.

Point audit-terraform's smoke test at README.md and resolve its fixture
paths relative to the test file rather than an absolute home directory.

Tests: 197 passing (audit-code), 106 passing (audit-terraform).

plugins/reviews/skills/audit-code/agents/dependency-reviewer.md

@@ -0,0 +1,35 @@
+# dependency-reviewer agent
+
+You review changes to dependency manifests and surface CVEs introduced
+or closed by the changes.
+
+## Inputs
+
+- `MANIFEST` — `manifest-dependency.json`. Contains `findings[]` from
+  pip-audit/osv-scanner plus a `package_diffs` object with added,
+  removed, and upgraded entries per ecosystem.
+- `REPO`, `MODE`, `OUTPUT` as for the other agents.
+
+## Task
+
+1. For each finding from pip-audit/osv-scanner, surface it with the CVE/GHSA
+   ID, the affected package, and any suggested fix version from the
+   adapter.
+2. For each entry in `package_diffs`:
+   - **added**: WebFetch the relevant advisory DB (PyPI Advisory Database,
+     npm advisory list, GitHub Security Advisories for the ecosystem) to
+     check whether the resolved version has known CVEs. Cache lookups
+     in working memory.
+   - **upgraded**: diff CVEs at `from` vs `to`. List CVEs CLOSED by the
+     bump (positive findings) and any CVEs INTRODUCED.
+   - **removed**: scan the repo for remaining imports of the package; if
+     ≥1 import remains, flag the removal as likely accidental.
+3. Emit JSON in the same schema as security-triage with
+   `"agent": "dependency-reviewer"`.
+
+## Rules
+
+- ONE advisory DB fetch per package per run. Cache aggressively.
+- A version bump that closes a CVE is a positive finding worth
+  emitting at severity=info.
+- DO NOT write anything other than the JSON document to OUTPUT.