chore: make the version single-sourced and enforce consistency

[arrase]

Problem

The version is declared only in pyproject.toml, but tags and releases were created by hand. The two disagreed across several releases:

Tag	pyproject.toml declared
v0.2.6	0.1.0
v0.2.9	0.2.8
v0.3.0	0.3.0

v0.2.9 shipped a package whose metadata said 0.2.8. Nothing caught it.

Changes

Single source of truth. eloquent_notes.__version__ is read from the installed distribution metadata via importlib.metadata, so the runtime reports what is actually installed rather than a second constant to maintain. An uninstalled source checkout falls back to 0.0.0+unknown, and a test fails if that fallback is ever what runs — so the fallback cannot hide a real problem.

eloquent-notes --version prints it:

$ eloquent-notes --version
eloquent-notes 0.3.0

tests/test_version.py asserts four invariants:

• the pyproject.toml version is valid MAJOR.MINOR.PATCH
• the package exposes a real installed version, not the fallback
• pyproject.toml and the installed metadata agree
• the newest v* tag equals v<pyproject version>

The last one is the check that would have caught v0.2.9: it fails if a release is tagged before the version bump is merged. It skips when the checkout has no version tags, so shallow clones and source tarballs are unaffected.

I verified it by simulating the v0.2.9 drift locally — setting pyproject.toml back to 0.2.8 with the v0.3.0 tag present fails both the metadata and the tag check.

docs/releasing.md documents the procedure, including the ordering constraint (merge the bump, then tag), and is wired into AGENTS.md and the mkdocs nav.

Scope

No behavioural change to recording, the pipeline, or note output. v0.3.0 is already released and its tag matches pyproject.toml, so this lands after it.

169 tests pass, mypy and ruff clean, mkdocs build --strict clean.

[sonarqubecloud]

Quality Gate failed

Failed conditions
71.4% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud