feat: model repo standards and detect prefix collisions

Implement RMGR-WP-0004 T01 (flavor, required files, anti-patterns from
canon) and T08 (prefix ownership registry plus uniqueness scan).
rmgr conform and rmgr prefix-uniqueness are detection only.

src/repo_manager/prefix_registry.py

@@ -0,0 +1,112 @@
+"""Fleet workplan-prefix ownership and uniqueness (RMGR-WP-0004-T08)."""
+
+from __future__ import annotations
+
+from collections import defaultdict
+from pathlib import Path
+from typing import Any
+
+import yaml
+
+from repo_manager.standards import FLAVOR_MARKER_PREFIX, workplan_ids
+
+DEFAULT_REGISTRY = Path(__file__).resolve().parents[2] / "config" / "workplan-prefix-registry.yaml"
+
+
+def load_registry(path: Path | None = None) -> dict[str, Any]:
+    target = path or DEFAULT_REGISTRY
+    data = yaml.safe_load(target.read_text(encoding="utf-8")) or {}
+    if not isinstance(data, dict):
+        return {}
+    return data
+
+
+def _skip_dir(name: str) -> bool:
+    return name.startswith(".") or name in {
+        "node_modules",
+        "__pycache__",
+        ".venv",
+        "venv",
+        "archive",
+    }
+
+
+def iter_repo_roots(root: Path) -> list[Path]:
+    root = root.resolve()
+    if (root / "workplans").is_dir() and (root / ".git").exists():
+        return [root]
+    found: list[Path] = []
+    if not root.is_dir():
+        return found
+    for child in sorted(root.iterdir()):
+        if not child.is_dir() or _skip_dir(child.name):
+            continue
+        if (child / "workplans").is_dir():
+            found.append(child)
+    return found
+
+
+def scan_prefixes(root: Path, *, registry_path: Path | None = None) -> dict[str, Any]:
+    """Detect prefix sharing, identifier reuse, flavor-derived prefixes, number reuse.
+
+    Detection only. Does not rewrite files or remediations.
+    """
+    registry = load_registry(registry_path)
+    owners = {str(k): str(v) for k, v in (registry.get("owners") or {}).items()}
+    retired = {str(item.get("prefix")) for item in registry.get("retired") or [] if item.get("prefix")}
+
+    by_prefix: dict[str, set[str]] = defaultdict(set)
+    by_id: dict[str, list[tuple[str, str]]] = defaultdict(list)
+    numbers: dict[tuple[str, str], list[str]] = defaultdict(list)
+
+    repos_scanned = []
+    for repo in iter_repo_roots(root):
+        repos_scanned.append(repo.name)
+        for rel, prefix, number in workplan_ids(repo):
+            by_prefix[prefix].add(repo.name)
+            ident = f"{prefix}-{number}"
+            by_id[ident].append((repo.name, rel))
+            numbers[(repo.name, prefix)].append(number)
+
+    shared = {
+        prefix: sorted(repos)
+        for prefix, repos in sorted(by_prefix.items())
+        if len(repos) > 1
+    }
+    reused = {
+        ident: [{"repo": repo, "path": path} for repo, path in entries]
+        for ident, entries in sorted(by_id.items())
+        if len({repo for repo, _path in entries}) > 1 or len(entries) > 1
+    }
+    flavor_derived = sorted(repo for repo in by_prefix.get(FLAVOR_MARKER_PREFIX, []))
+    retired_in_use = {
+        prefix: sorted(repos)
+        for prefix, repos in by_prefix.items()
+        if prefix in retired
+    }
+    owner_mismatch = {
+        prefix: {"declared_owner": owners[prefix], "seen_in": sorted(repos)}
+        for prefix, repos in by_prefix.items()
+        if prefix in owners and any(repo != owners[prefix] for repo in repos)
+    }
+
+    reused_numbers = []
+    for (repo, prefix), nums in numbers.items():
+        seen: set[str] = set()
+        for num in nums:
+            if num in seen:
+                reused_numbers.append({"repo": repo, "prefix": prefix, "number": num})
+            seen.add(num)
+
+    return {
+        "ok": not shared and not flavor_derived and not reused_numbers and not reused,
+        "root": str(root.resolve()),
+        "repos_scanned": repos_scanned,
+        "shared_prefixes": shared,
+        "reused_identifiers": reused,
+        "flavor_derived_prefix": flavor_derived,
+        "retired_in_use": retired_in_use,
+        "owner_mismatch": owner_mismatch,
+        "reused_numbers_in_repo": reused_numbers,
+        "detection_only": True,
+    }