chore(release): 0.3.1

[caohy1988]

Release PR for 0.3.1 — focused follow-up to 0.3.0 that publishes the periodic-materialization production path.

Why 0.3.1 (patch) and not 0.4.0 (minor)

0.3.0 cut on 2026-05-15 (#163). Between that cut and now, six PRs landed on main:

PR	Type	Customer-visible?
#162	new CLI (bqaa-materialize-window)	yes — new console script, not on PyPI yet
#164	tests (live BQ idempotency proof)	no
#165	examples (Cloud Run Job + Scheduler)	yes — example directory
#166	examples (live deploy + IAM fixes)	yes — example directory
#167	fix (materialize_window empty-extraction)	yes — behavior change
#168, #170	docs (customer playbook + release-notes index)	yes — playbook

A strict semver read would call #162 a minor bump (new public API surface). Going with patch instead because:

1. The bqaa-materialize-window CLI was originally meant to ship in 0.3.0 — it merged the same week and missed the cut by branch timing, not by design. The Unreleased block has been waiting for a 0.3.x cleanup.
2. Everything else in scope is either a bug fix (fix(materialize_window): empty-extraction sessions are not ok (#166 follow-up) #167), examples (examples(migration_v5): Cloud Run Job + Scheduler periodic materialization #165, examples(migration_v5): live Cloud Run deploy verification + IAM fixes (#165 follow-up) #166), or docs (docs(migration_v5): customer playbook for periodic materialization (#167 follow-up) #168, docs: add migration v5 periodic materialization release notes #170).
3. There are no breaking changes, no new SDK Python APIs beyond the CLI surface.

If a maintainer would rather cut this as 0.4.0, the diff swap is two characters in pyproject.toml and the section header in CHANGELOG.md.

What this PR does

• Bumps pyproject.toml version from 0.3.0 to 0.3.1.
• Renames the existing ## [Unreleased] section to ## [0.3.1] - 2026-05-18 in CHANGELOG.md.
• Adds a Release highlights summary scoped to the periodic-materialization production path.
• Adds ### Added entries for bqaa-materialize-window (cli: add bqaa-materialize-window for scheduled graph refresh (#161 MVP) #162) and the Cloud Run Job + Cloud Scheduler example (examples(migration_v5): Cloud Run Job + Scheduler periodic materialization #165, examples(migration_v5): live Cloud Run deploy verification + IAM fixes (#165 follow-up) #166) — both shipped post-0.3.0 but were never release-noted.
• Adds a ### Fixed entry for fix(materialize_window): empty-extraction sessions are not ok (#166 follow-up) #167 with the operator-visible behavior change called out (alerts on jsonPayload.ok=false are now sufficient; no rows_materialized == {} second-line check needed).
• Keeps the existing playbook entry for docs(migration_v5): customer playbook for periodic materialization (#167 follow-up) #168.
• Re-opens an empty ## [Unreleased] section above 0.3.1 for post-release work.

No code changes. All entries point to PRs already merged on main.

Verification

• pyproject.toml parses: python3 -c "import tomllib; print(tomllib.loads(open('pyproject.toml').read())['project']['version'])" → 0.3.1.
• CHANGELOG.md headers are monotonic + well-formed: [Unreleased] → [0.3.1] - 2026-05-18 → [0.3.0] - 2026-05-15 → [0.2.3] - 2026-04-27 → [0.2.2] - 2026-04-24 → [0.2.1].
• Diff is exactly two files (+97 / -1 lines), no other modifications.

Post-merge

Once this lands on main:

1. Tag v0.3.1 on the merge commit.
2. Trigger the PyPI publish flow (whichever release pipeline the repo uses — confirm with a maintainer before tagging).
3. Verify bqaa-materialize-window --help works after a clean pip install bigquery-agent-analytics==0.3.1 on a fresh venv.

Test plan

• pyproject.toml parses; version is 0.3.1.
• CHANGELOG.md headers are well-formed, monotonic, and dated.
• Empty [Unreleased] placeholder re-opened above 0.3.1.
• No code changes — only pyproject.toml and CHANGELOG.md touched.
• CI green on this PR.
• Post-merge: pip install bigquery-agent-analytics==0.3.1 → bqaa-materialize-window --help renders.