Phase 6: `--check` should print a unified diff in `gen_docs.py` and `sync_agents_md.py`

[thomaschristory]

Both drift-detection scripts only print the names of drifted files when their --check flag fires. CI logs say "out of date; re-run …" without showing what's actually different — a maintainer has to clone the PR locally and re-run the generator to see the diff.

Current state

• scripts/gen_docs.py (5b): on drift, prints reference pages drifted (re-run \python scripts/gen_docs.py`): ['cli.md', ...]`. No content diff.
• scripts/sync_agents_md.py (5d): on drift, prints AGENTS.md is out of date; run \uv run python scripts/sync_agents_md.py` and commit the result.` No content diff.

Proposed change

Add a difflib.unified_diff step before the error message in both scripts. Format:

$ uv run python scripts/sync_agents_md.py --check
AGENTS.md is out of date. Diff (expected vs current):
--- expected
+++ AGENTS.md
@@ -42,3 +42,4 @@
 line that's still there
-removed line
+added line
+other added line

run `uv run python scripts/sync_agents_md.py` and commit the result.

For gen_docs.py: print the diff per drifted file, capped at ~40 lines per file to avoid log spam if a generator change rewrote everything.

Why deferred from v1

Both --check flags work correctly; they just produce sparse output. CI gates fire as expected. This is a maintainer-ergonomics improvement that costs ~10 lines per script.

Acceptance

• Both --check flags print a difflib.unified_diff-formatted diff to stderr before exiting non-zero.
• New --no-diff flag preserves the current behavior for callers that want only the summary line.
• Test coverage: each script's existing test suite gains a "drift produces a diff" case.