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.

.agent/reports/08_story_dark_tech_theme.a1.validate

@@ -0,0 +1,34 @@
+
+-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
+================================ tests coverage ================================
+_______________ coverage: platform linux, python 3.13.13-final-0 _______________
+
+Name                     Stmts   Miss  Cover
+--------------------------------------------
+app/__init__.py              1      0   100%
+app/api/__init__.py          0      0   100%
+app/api/chat.py             84      0   100%
+app/api/docs.py             12      0   100%
+app/api/health.py           10      0   100%
+app/api/suggestions.py       8      0   100%
+app/config.py               31      0   100%
+app/core/__init__.py         0      0   100%
+app/core/debugging.py       29      2    93%
+app/core/logging.py         13      0   100%
+app/db.py                   21      0   100%
+app/main.py                 28      0   100%
+app/models.py               40      0   100%
+app/rag/__init__.py          0      0   100%
+app/rag/chunker.py         126      4    97%
+app/rag/importer.py        136      5    96%
+app/rag/llm.py              94      0   100%
+app/rag/prompts.py          15      0   100%
+app/rag/retriever.py        49      0   100%
+app/rag/suggestions.py      27      0   100%
+app/schemas.py              32      0   100%
+--------------------------------------------
+TOTAL                      756     11    99%
+coverage gate: app/ 99% (>90%) OK
+All checks passed!
+0 errors, 0 warnings, 0 informations
+validation OK