feat(plugin): every session says which plugin version is executing

Twice a shipped fix failed to reach a live install, and both times the only
detector was the operator saying "I don't think it updated" (#2198, #2209).

The reason it is hard to see: an install has two halves and only one
self-updates. The marketplace clone pulls on its own; the CACHE is what
executes and refreshes only when the manifest version changes. So inspecting
the clone shows the fix present while the broken copy keeps running — the
obvious debugging move actively misleads.

The SessionStart context now names the version it is running. That makes "what
is actually executing?" answerable from the transcript rather than by
archaeology in the cache directory.

DELIBERATELY SMALLER THAN THE ISSUE PROPOSED. #2220 recommended reporting the
version to the server, storing last-seen per user, and surfacing it in
Settings. That is three surfaces and a migration to answer a question the
session can answer about itself. Per the operator's framing on #2338 — "I'm
afraid of building another integration between two more surfaces, you being
able to notice is enough" — the visibility is the deliverable, not the
plumbing.

It also lands the issue's own caveat, which the server-side design could not:
the state most needing diagnosis is the one where credentials never arrive,
and there the dynamic tier does not run at all. This marker is keyless and
networkless, so it still appears — verified against both the unconfigured and
unreachable-instance paths.

CI pins it, asserting WITHOUT credentials for the same reason. Manifest 0.1.22
-> 0.1.23, because a shipped hook changed and an unbumped manifest is precisely
the failure this commit is about.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

scripts/check_plugin.py

@@ -307,6 +307,44 @@ def check_local_prior_art_needs_no_instance() -> None:
         ok("prior-art local arm: answers with no instance configured")
 
 
+def check_session_context_reports_its_version() -> None:
+    """The SessionStart context must name the plugin version it is running.
+
+    An install has two halves and only one self-updates: the marketplace clone
+    pulls on its own, while the CACHE is what executes and refreshes only when
+    the manifest version changes. So a shipped fix can sit unreached while
+    inspecting the clone shows it present — the obvious debugging move
+    misleads, and twice the only detector was a human saying "I don't think it
+    updated" (#2209, #2220).
+
+    Asserted WITHOUT credentials on purpose. The state most needing diagnosis
+    is the one where the token never arrives, and a marker that vanished there
+    would be missing exactly when it is wanted.
+    """
+    script = HOOKS_DIR / "scribe_session_context.sh"
+    if not script.is_file() or not shutil.which("jq"):
+        skip("version marker: hook or jq missing")
+        return
+
+    manifest_v = manifest_version()
+    if manifest_v is None:
+        fail("version marker: could not read the manifest version")
+        return
+
+    try:
+        proc = _run_hook(script, json.dumps({"source": "startup"}), {})
+    except subprocess.TimeoutExpired:
+        fail("version marker: hook hung")
+        return
+    if proc.returncode != 0:
+        fail(f"version marker: hook exited {proc.returncode}")
+    elif manifest_v not in proc.stdout:
+        fail(f"version marker: session context never names v{manifest_v} — "
+             f"a stale install would be undetectable from the transcript")
+    else:
+        ok(f"version marker: session context reports v{manifest_v}, no credentials needed")
+
+
 def _git(*args: str) -> tuple[int, str]:
     proc = subprocess.run(
         ["git", *args], capture_output=True, text=True, cwd=ROOT
@@ -399,6 +437,7 @@ def main() -> int:
     check_shellcheck()
     check_fail_open()
     check_local_prior_art_needs_no_instance()
+    check_session_context_reports_its_version()
     if not args.no_version:
         check_version_bump(args.base)