feat(pair-corpus-honesty): deterministic provenance fingerprint at scan time

Task 6.1.

- provenance.py: classify a checkout as human / agent / mixed / unknown
  from deterministic signals only (.claude/, AGENTS.md, Lovable meta,
  agent git trailers on a bounded commit sample); no model involved.
- cli scan runs a `provenance` stage; the manifest records the result.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

src/openultrasast/provenance.py

@@ -0,0 +1,117 @@
+"""Deterministic provenance fingerprint: does this tree look agent-authored?
+
+pair-corpus-honesty Requirement 6. Signals are files and directories that agent
+tooling leaves behind, generation markers in text, and git co-author trailers
+over a bounded commit sample. No model is consulted. ``synthetic`` is never
+produced by a scan; it is a catalog declaration only.
+"""
+
+from __future__ import annotations
+
+import re
+import subprocess
+from dataclasses import dataclass
+from pathlib import Path
+
+PROVENANCE_VALUES = ("human", "agent", "mixed", "synthetic")
+AGENT_SHARE_THRESHOLD = 0.85  # VibeApps method: >=85% of sampled commits carry an agent signal
+DEFAULT_SAMPLE_COMMITS = 200
+
+_AGENT_PATHS = (
+    ".claude",
+    "CLAUDE.md",
+    "AGENTS.md",
+    ".cursor",
+    ".cursorrules",
+    ".windsurf",
+    ".windsurfrules",
+    ".github/copilot-instructions.md",
+    ".codex",
+    ".jules",
+    ".kiro",
+    ".opencode",
+)
+_MARKERS = (
+    ("Generated with Claude Code", "marker:claude-code"),
+    ('<meta name="author" content="Lovable"', "marker:lovable"),
+    ("lovable.dev", "marker:lovable"),
+    ("bolt.new", "marker:bolt"),
+    ("Made with Cursor", "marker:cursor"),
+)
+_TRAILER_RE = re.compile(
+    r"^(?:co-authored-by|generated-by|signed-off-by):\s*.*?(claude|codex|copilot|cursor|devin|jules|gemini|windsurf|aider|openai)\b",
+    re.IGNORECASE | re.MULTILINE,
+)
+_GENERATED_RE = re.compile(r"generated with (claude code|codex|cursor|copilot|devin|jules)", re.IGNORECASE)
+_MARKER_FILES = ("index.html", "README.md", "readme.md", "package.json")
+
+
+@dataclass(frozen=True)
+class Provenance:
+    value: str
+    signals: tuple[str, ...]
+    sampled_commits: int = 0
+    agent_commits: int = 0
+
+    def to_dict(self) -> dict[str, object]:
+        return {
+            "value": self.value,
+            "signals": list(self.signals),
+            "sampled_commits": self.sampled_commits,
+            "agent_commits": self.agent_commits,
+        }
+
+
+def fingerprint(root: Path, *, git: bool = True, sample_commits: int = DEFAULT_SAMPLE_COMMITS) -> Provenance:
+    """Classify ``root`` from deterministic signals only. Never raises into a scan."""
+    signals: list[str] = []
+    for relative in _AGENT_PATHS:
+        if (root / relative).exists():
+            signals.append(f"path:{relative}")
+    for name in _MARKER_FILES:
+        path = root / name
+        if not path.is_file():
+            continue
+        try:
+            text = path.read_text(errors="ignore")[:200_000]
+        except OSError:
+            continue
+        for needle, label in _MARKERS:
+            if needle in text and label not in signals:
+                signals.append(label)
+    sampled = agent = 0
+    if git:
+        sampled, agent, git_signal = _git_trailers(root, sample_commits)
+        if git_signal:
+            signals.append(git_signal)
+    if sampled and agent / sampled >= AGENT_SHARE_THRESHOLD:
+        value = "agent"
+    elif any(not signal.startswith("git:") for signal in signals) or agent:
+        value = "mixed"
+    else:
+        value = "human"
+    return Provenance(value=value, signals=tuple(signals), sampled_commits=sampled, agent_commits=agent)
+
+
+def _git_trailers(root: Path, sample_commits: int) -> tuple[int, int, str]:
+    """(sampled, agent-signalled, signal-label). Missing git is a signal, not an error."""
+    if sample_commits <= 0:
+        return 0, 0, ""
+    try:
+        completed = subprocess.run(
+            ["git", "-C", str(root), "log", f"-{sample_commits}", "--format=%H%x00%B%x1e"],
+            capture_output=True,
+            text=True,
+            timeout=20,
+            check=False,
+        )
+    except (OSError, subprocess.SubprocessError):
+        return 0, 0, "git:unavailable"
+    if completed.returncode != 0:
+        return 0, 0, "git:unavailable"
+    entries = [entry for entry in completed.stdout.split("\x1e") if entry.strip()]
+    sampled = len(entries)
+    agent = sum(1 for entry in entries if _TRAILER_RE.search(entry) or _GENERATED_RE.search(entry))
+    if not sampled:
+        return 0, 0, "git:no-commits"
+    return sampled, agent, f"git:trailers {agent}/{sampled}"