feat(my-deepagent): v0.2 PR #1 — Postgres migration (ahead of M8-Py FastAPI)

Switches the production backing store from SQLite to PostgreSQL 16, per DR-2.
The migration trigger is two concurrent writers on the my-deepagent ORM
tables — which first appears with FastAPI (M8-Py). Doing the cut now keeps
the surface area small while M8-Py is still planning.

Production deps: `asyncpg`, `psycopg[binary]`, `langgraph-checkpoint-postgres`.
Test deps: `aiosqlite` (the bulk of unit + integration tests stay on sqlite
tmp_path for speed; the E2E suite and the new checkpointer tests exercise
the live Postgres path).

Highlights
- `persistence/db.py`: dialect-aware connect listener. SQLite still gets
  WAL + busy_timeout=5000 + foreign_keys=ON; Postgres gets `SET TIME ZONE 'UTC'`.
  Added `Database.dialect_name` + `drop_schema` (test-only).
- `persistence/checkpointer.py`: SqliteSaver → AsyncPostgresSaver. API is
  now async (`async with`) and takes a connection string. SQLAlchemy URL
  prefixes (`+asyncpg`, `+psycopg`) are auto-stripped to a plain libpq DSN
  (`_to_psycopg_dsn` helper, 4 unit tests).
- `persistence/upsert.py` (new): `insert_for(session)` — dialect-aware UPSERT
  helper. Picks `postgresql.insert` or `sqlite.insert` based on the bound
  engine. Replaces 5 hardcoded `sqlite_insert` call sites in `budget.py`,
  `recovery.py`, `cli/doctor.py`.
- `persistence/models.py`: `RunRow` partial unique index declares both
  `postgresql_where=` and `sqlite_where=` for cross-dialect correctness.
- `config.py`: default `database_url` now
  `postgresql+asyncpg://devflow:devflow@localhost:55432/mydeepagent`. v3
  `devflow` DB preserved untouched; v4 lives in a fresh `mydeepagent` DB.
- `cli/doctor.py` check 8: dialect-aware DB liveness probe. Postgres path
  runs `SELECT 1` (pg_isready equivalent); SQLite keeps `PRAGMA integrity_check`.
- `alembic/env.py`: env-aware URL resolution (`MYDEEPAGENT_DATABASE_URL` >
  `DATABASE_URL` > default). Async driver prefixes are mapped to the sync
  equivalents alembic needs.
- `alembic/versions/9f2a6c79667e_v0_2_baseline_schema_postgres.py` (new):
  fresh baseline autogenerated against live Postgres. Old SQLite migrations
  (`79945fdc2649`, `839f2233e346`) deleted — v0.2 starts a clean history.
- `tests/conftest.py` (new): `pg_db_url` async fixture creates a fresh DB
  per test against docker-compose `devflow-postgres` and drops it on
  teardown after terminating lingering backends.
- `tests/integration/test_checkpointer.py`: rewritten for AsyncPostgresSaver
  (4 pure DSN-converter unit tests + 3 async context-manager integration tests).
- `tests/integration/test_e2e_workflow.py`: switched to `pg_db_url`. Real
  OpenRouter E2E now exercises the production Postgres path end-to-end.

Recovery
- Previous SQLite database at the platformdirs data_dir is NOT auto-migrated;
  v0.1.0 was the only release that wrote to it. Set
  `MYDEEPAGENT_DATABASE_URL=sqlite+aiosqlite:///<path>` to read it.
- The v3 `devflow` Postgres DB is preserved untouched (separate database
  name); to inspect: `psql -h localhost -p 55432 -U devflow -d devflow`.

Gates
- ruff check + ruff format --check + mypy --strict: PASS (102 source files)
- pytest non-E2E: 576 PASS (5.46 s)
- pytest E2E real OpenRouter on Postgres: 1 PASS (122.93 s, ~$0.05/run)

--no-verify: lefthook still TS-only (deleted in 0e61b2d but still queryable
in git history).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

my-deepagent/pyproject.toml

@@ -4,7 +4,8 @@ version = "0.1.0"
 description = "Add your description here"
 requires-python = ">=3.12"
 dependencies = [
-    "aiosqlite>=0.20",
+    "asyncpg>=0.30",
+    "psycopg[binary]>=3.2",
     "alembic>=1.14",
     "greenlet>=3.0",
     "sqlalchemy[asyncio]>=2.0",
@@ -15,7 +16,7 @@ dependencies = [
     "langchain-core>=0.3.0,<2.0.0",
     "langchain-openai>=0.3.0,<2.0.0",
     "langgraph>=0.2.0",
-    "langgraph-checkpoint-sqlite>=2.0.0",
+    "langgraph-checkpoint-postgres>=2.0.0",
     "openai>=1.0.0",
     "platformdirs>=4.9",
     "prompt-toolkit>=3.0",
@@ -46,6 +47,12 @@ markers = [
 
 [dependency-groups]
 dev = [
+    # aiosqlite is a TEST-ONLY dependency: production runs on Postgres
+    # (asyncpg, see [project.dependencies]) but the bulk of the test suite uses
+    # sqlite+aiosqlite tmp_path URLs for speed + isolation simplicity. Live
+    # Postgres validation happens via the E2E suite (real OpenRouter +
+    # docker-compose Postgres).
+    "aiosqlite>=0.20",
     "mypy>=1.13",
     "pre-commit>=4.0",
     "pytest>=8.3",