fix(#29): display commit SHA + content checksum on skill install

src/install.py

@@ -214,19 +214,27 @@ def install(repo, skills, install_all, targets, branch, list_only, yes):
     # Fetch and install
     installed_skills = []
     for skill_name in skill_names:
-        click.echo(f"  Fetching {skill_name}...", nl=False)
         skill = fetch_skill_from_repo(repo, skill_name, branch)
         if not skill:
-            click.echo(f" not found in {repo}")
+            click.echo(f"  ! {skill_name}: not found in {repo}", err=True)
             continue
 
         # Validate name once more before writing to disk (save_skill_file also validates)
         try:
             sanitize_skill_name(skill["name"])
         except ValueError as exc:
-            click.echo(f" skipped — invalid name: {exc}", err=True)
+            click.echo(f"  ! {skill_name}: skipped — invalid name: {exc}", err=True)
             continue
 
+        # Display integrity info for user audit (#29)
+        commit_sha = skill.pop("_commit_sha", None)
+        content_checksum = skill.pop("_content_checksum", None)
+        sha_display = commit_sha[:12] if commit_sha else "unknown"
+        checksum_short = content_checksum.split(":")[1][:16] if content_checksum else "?"
+        click.echo(
+            f"  Fetching {skill['name']} from {repo} @ {sha_display} (sha256:{checksum_short}...)"
+        )
+
         # Save to ~/.apc/skills/<name>/SKILL.md
         raw_content = skill.pop("_raw_content", skill.get("body", ""))
         save_skill_file(skill["name"], raw_content)
@@ -240,7 +248,7 @@ def install(repo, skills, install_all, targets, branch, list_only, yes):
         save_skills(merged)
 
         installed_skills.append(skill["name"])
-        click.echo(" ✓")
+        click.echo(f"  ✓ {skill['name']} installed")
 
     if installed_skills:
         click.echo(f"\n✓ Installed {len(installed_skills)} skill(s) to {', '.join(target_list)}")

src/skills.py

@@ -4,6 +4,7 @@
 tool's skill directory on sync.
 """
 
+import hashlib
 import re
 from pathlib import Path
 from typing import Any, Dict, List, Optional
@@ -16,6 +17,9 @@
 DEFAULT_BRANCH = "main"
 _GITHUB_TREE_API = "https://api.github.com/repos/{repo}/git/trees/{branch}?recursive=1"
 _GITHUB_RAW = "https://raw.githubusercontent.com/{repo}/{branch}/skills/{skill}/SKILL.md"
+_GITHUB_COMMITS_API = (
+    "https://api.github.com/repos/{repo}/commits?path=skills/{skill}/SKILL.md&per_page=1"
+)
 
 _SKILL_NAME_SAFE = re.compile(r"^[A-Za-z0-9][A-Za-z0-9_\-]{0,63}$")
 
@@ -105,14 +109,36 @@ def list_skills_in_repo(repo: str, branch: str = DEFAULT_BRANCH) -> List[str]:
     return sorted(names)
 
 
+def _fetch_skill_commit_sha(repo: str, skill_name: str) -> Optional[str]:
+    """Return the latest commit SHA for a skill file, or None on error.
+
+    Uses the GitHub Commits API with a path filter so we get the SHA of the
+    commit that last touched skills/<skill_name>/SKILL.md.  This is
+    displayed at install-time so users can audit the exact version (#29).
+    """
+    url = _GITHUB_COMMITS_API.format(repo=repo, skill=skill_name)
+    try:
+        resp = _safe_get(url)
+        if resp.status_code != 200:
+            return None
+        data = resp.json()
+        if isinstance(data, list) and data:
+            return data[0].get("sha")
+    except (httpx.HTTPError, Exception):
+        pass
+    return None
+
+
 def fetch_skill_from_repo(
     repo: str,
     skill_name: str,
     branch: str = DEFAULT_BRANCH,
 ) -> Optional[Dict[str, Any]]:
     """Fetch and parse a single skill from a GitHub repo.
 
-    Returns a skill dict (with _raw_content) or None if not found.
+    Returns a skill dict (with _raw_content and _commit_sha) or None if not found.
+    The _commit_sha field contains the SHA of the last commit that touched the
+    skill file — callers should display this so users can audit the download (#29).
     """
     url = _GITHUB_RAW.format(repo=repo, branch=branch, skill=skill_name)
     try:
@@ -130,6 +156,11 @@ def fetch_skill_from_repo(
     except ValueError:
         # Fall back to the URL-path component (already validated by list_skills_in_repo)
         safe_name = sanitize_skill_name(skill_name)
+
+    # Compute content checksum and fetch commit SHA for integrity display (#29)
+    content_checksum = "sha256:" + hashlib.sha256(resp.text.encode()).hexdigest()
+    commit_sha = _fetch_skill_commit_sha(repo, skill_name)
+
     return {
         "name": safe_name,
         "description": metadata.get("description", ""),
@@ -140,4 +171,6 @@ def fetch_skill_from_repo(
         "source_tool": "github",
         "source_repo": repo,
         "_raw_content": resp.text,
+        "_content_checksum": content_checksum,
+        "_commit_sha": commit_sha,
     }

tests/test_security_input_validation.py

@@ -204,11 +204,14 @@ def mock_get(url, follow_redirects=True, timeout=15):
 
             fetch_skill_from_repo("owner/repo", "my-skill", "main")
 
-        self.assertEqual(len(calls), 1)
-        self.assertFalse(
-            calls[0]["follow_redirects"],
-            "follow_redirects must be False to prevent SSRF",
-        )
+        # 2 calls: (1) raw SKILL.md content, (2) GitHub Commits API for SHA (#29)
+        # All calls must have follow_redirects=False
+        self.assertGreaterEqual(len(calls), 1)
+        for call in calls:
+            self.assertFalse(
+                call["follow_redirects"],
+                "follow_redirects must be False to prevent SSRF",
+            )
 
 
 if __name__ == "__main__":