ci: enhance coverage with feature matrix, postgres, and E2E (#523)

* ci: enhance coverage workflow with feature matrix, postgres, and E2E

Replace single-config coverage job with a multi-job pipeline:

- Mirror test.yml's 3-config feature matrix (all-features, default, libsql-only)
- Add PostgreSQL service (pgvector/pgvector:pg16) with migrations for
  postgres configs so integration tests actually run instead of skipping
- Add E2E coverage job using cargo-llvm-cov instrumented binary with
  Playwright browser tests
- Add coverage-gate roll-up job for branch protection
- Upload per-config flags to Codecov (all-features, default, libsql-only, e2e)
- Forward LLVM coverage env vars in E2E conftest.py so profraw data
  lands where cargo-llvm-cov report expects it

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* fix: address PR review feedback on coverage workflow

- Avoid setting DATABASE_URL to empty string for libsql-only config;
  use $GITHUB_ENV conditional step so the var is unset entirely
- Add set -euo pipefail and psql -v ON_ERROR_STOP=1 to migrations
  so SQL errors fail the job immediately

[skip-regression-check]

Co-Authored-By: Claude Opus 4.6 <[email protected]>

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>

tests/e2e/conftest.py

@@ -98,6 +98,13 @@ async def ironclaw_server(ironclaw_binary, mock_llm_server):
         # Prevent onboarding wizard from triggering
         "ONBOARD_COMPLETED": "true",
     }
+    # Forward LLVM coverage instrumentation env vars when present
+    # (allows cargo-llvm-cov to collect profraw data from E2E runs)
+    for key in ("LLVM_PROFILE_FILE", "CARGO_LLVM_COV", "CARGO_LLVM_COV_SHOW_ENV",
+                "CARGO_LLVM_COV_TARGET_DIR"):
+        val = os.environ.get(key)
+        if val is not None:
+            env[key] = val
     proc = await asyncio.create_subprocess_exec(
         ironclaw_binary, "--no-onboard",
         stdin=asyncio.subprocess.DEVNULL,