fix(agent-context): discover nested plans in Python port mtime fallback (#3734)

The Python port of update-agent-context reintroduced a one-level plan
scan (specs/*/plan.md) in its mtime fallback, while the Bash and
PowerShell ports search recursively (specs/**/plan.md) per the fix for
issue #3024. The three ports were therefore not in parity: for nested
scoped layouts such as specs/<scope>/<feature>/plan.md, the Python port
found no plan and omitted the plan link from the managed context section.

Switch the fallback to `(root / "specs").rglob("plan.md")` and update the
module docstring to match the documented recursive-discovery contract.
Add a parity regression test covering the nested layout (it fails on the
one-level glob and passes with the recursive scan).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

extensions/agent-context/scripts/python/update_agent_context.py

@@ -11,8 +11,9 @@ Usage: update_agent_context.py [plan_path]
 
 When ``plan_path`` is omitted, the script derives it from
 ``.specify/feature.json`` (written by /speckit-specify). Falls back to the most
-recently modified ``specs/*/plan.md`` only when feature.json is absent or its
-plan does not exist yet.
+recently modified ``plan.md`` anywhere under ``specs/`` (including nested scoped
+layouts such as ``specs/<scope>/<feature>/plan.md``) only when feature.json is
+absent or its plan does not exist yet.
 """
 
 from __future__ import annotations
@@ -173,7 +174,7 @@ def _resolve_plan_path(project_root: str) -> str:
     if not plan_path:
         root = Path(project_root).resolve()
         plans = sorted(
-            (root / "specs").glob("*/plan.md"),
+            (root / "specs").rglob("plan.md"),
             key=lambda p: p.stat().st_mtime,
             reverse=True,
         )