chore(agent): track .agent/ planning tree in git

Remove the blanket .agent/ gitignore so the phase roadmap, user
stories, reports, and PLAN.md are versioned with the code. Only
runtime artifacts (.agent/phase-sessions/, .agent/pipeline.log)
remain ignored. Update AGENTS.md git protocol rule to match.

.gitignore

@@ -16,8 +16,9 @@ htmlcov/
 .env.*
 !.env.example
 
-# --- Planning / agent pipeline state (committed on demand with `git add -f .agent/...`) ---
-.agent/
+# --- Planning / agent pipeline state (the .agent/ tree is tracked; only runtime artifacts ignored) ---
+.agent/phase-sessions/
+.agent/pipeline.log
 
 # --- Frontend build artifacts (sources live in frontend/, builds are local-only) ---
 node_modules/