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/scripts/git_diff.py

@@ -0,0 +1,122 @@
+"""Git diff scanning with origin-base resolution and defensive flags."""
+from __future__ import annotations
+
+import re
+import subprocess
+
+
+_HUNK_RE = re.compile(r"^@@ -\d+(?:,\d+)? \+(\d+)(?:,(\d+))? @@")
+
+
+def resolve_default_branch(repo: str) -> str:
+    try:
+        r = subprocess.run(
+            ["git", "-C", repo, "symbolic-ref", "refs/remotes/origin/HEAD"],
+            capture_output=True, text=True, check=False,
+        )
+        if r.returncode == 0:
+            return r.stdout.strip().rsplit("/", 1)[-1]
+    except FileNotFoundError:
+        pass
+    for candidate in ("main", "master"):
+        r = subprocess.run(
+            ["git", "-C", repo, "rev-parse", f"origin/{candidate}"],
+            capture_output=True, text=True, check=False,
+        )
+        if r.returncode == 0:
+            return candidate
+    return "main"
+
+
+def resolve_base_ref(repo: str, branch: str) -> str:
+    """Return the diff base for `branch`, fetching origin first."""
+    try:
+        subprocess.run(
+            ["git", "-C", repo, "fetch", "--quiet", "--no-tags",
+             "origin", branch],
+            capture_output=True, text=True, check=False, timeout=60,
+        )
+    except (FileNotFoundError, subprocess.TimeoutExpired):
+        pass
+    check = subprocess.run(
+        ["git", "-C", repo, "rev-parse", "--verify", "--quiet",
+         f"origin/{branch}"],
+        capture_output=True, text=True, check=False,
+    )
+    return f"origin/{branch}" if check.returncode == 0 else branch
+
+
+def _run_git_diff(repo: str, base: str, head: str) -> str:
+    result = subprocess.run(
+        [
+            "git", "-C", repo,
+            "-c", "diff.noprefix=false",
+            "-c", "color.ui=never",
+            "diff", "--no-ext-diff", "--unified=0", f"{base}...{head}",
+        ],
+        capture_output=True, text=True, check=False,
+    )
+    if result.returncode != 0:
+        raise RuntimeError(f"git diff failed: {result.stderr.strip()}")
+    return result.stdout
+
+
+def _iter_file_blocks(diff_text: str):
+    current: str | None = None
+    buf: list[str] = []
+    for line in diff_text.splitlines():
+        if line.startswith("diff --git "):
+            if current is not None:
+                yield current, "\n".join(buf)
+            current = None
+            buf = []
+        elif line.startswith("+++ b/"):
+            current = line[len("+++ b/"):]
+        if current is not None:
+            buf.append(line)
+    if current is not None:
+        yield current, "\n".join(buf)
+
+
+def _added_ranges(block: str) -> list[tuple[int, int]]:
+    ranges: list[tuple[int, int]] = []
+    cur: int | None = None
+    start: int | None = None
+    end: int | None = None
+    for line in block.splitlines():
+        m = _HUNK_RE.match(line)
+        if m:
+            if start is not None:
+                ranges.append((start, end))  # type: ignore[arg-type]
+                start = end = None
+            cur = int(m.group(1))
+            continue
+        if cur is None:
+            continue
+        if line.startswith("+") and not line.startswith("+++"):
+            if start is None:
+                start = cur
+            end = cur
+            cur += 1
+        elif line.startswith("-") and not line.startswith("---"):
+            continue
+        else:
+            if start is not None:
+                ranges.append((start, end))  # type: ignore[arg-type]
+                start = end = None
+            cur += 1
+    if start is not None:
+        ranges.append((start, end))  # type: ignore[arg-type]
+    return ranges
+
+
+def changed_files_with_ranges(
+    repo: str, base: str, head: str,
+) -> list[tuple[str, list[tuple[int, int]]]]:
+    """Return [(path, added_line_ranges), ...] for every changed file."""
+    out: list[tuple[str, list[tuple[int, int]]]] = []
+    for path, block in _iter_file_blocks(_run_git_diff(repo, base, head)):
+        ranges = _added_ranges(block)
+        if ranges:
+            out.append((path, ranges))
+    return out