ci: rename test job to continuous-integration to match main branch protection

[BerryUIKI]

Summary

Fixes the perpetually-waiting CI on main release PRs (#60).

Root cause

main branch protection requires a status check named continuous-integration, but test.yml's job is named test with a 3.12 matrix — GitHub reports it as test (3.12). The required continuous-integration check never appears, so the PR stays "waiting" even though every check actually passes.

Fix

• test.yml: job renamed test → continuous-integration, matrix removed (Python 3.12 pinned directly) — without a matrix, GitHub reports the check under the bare job name, matching the protection config exactly.
• CHANGELOG.md: [Unreleased] entry added.

What these workflows do

• Test (test.yml): Python 3.12 + uv → ruff check (lint, advisory) + pytest (full suite).
• pr-source-guard (pr-source-guard.yml): blocks PRs to main from any branch other than dev / hotfix/*.

Note on merge order

This PR and #61 (README consolidation) both touch the [Unreleased] CHANGELOG section. Whichever merges second may hit a trivial CHANGELOG conflict — resolve by keeping both entries.