feat(build): match local Python version by default

[deanq]

What

Makes flash build / flash deploy resolve the app Python version from the local interpreter (sys.version_info) when no --python-version override and no per-resource python_version is declared — replacing the hardcoded 3.12 fallback. An unsupported local interpreter (e.g. 3.9, 3.14) raises an actionable error. The build prints the resolved version and its source (matched local interpreter / --python-version override / declared on resource X).

Why

Supersedes #330. That branch was cut from the #346 merge (since reverted via #350) and its files were corrupted by a bad conflict resolution. The "add 3.13" half of #330 already landed on main independently; the only genuinely-unmerged piece was this match-local behavior, extracted here cleanly against current main.

Resolution order (unchanged except the fallback):

1. --python-version override (validated; must match any per-resource declarations)
2. Exactly one distinct per-resource python_version
3. Local interpreter (was: hardcoded DEFAULT_PYTHON_VERSION)

Test plan

• manifest.py::_reconcile_python_version: new TestReconcileLocalInterpreter — parametrized local-match (3.10–3.13), unsupported-local raises with actionable message, override/declaration are not shadowed by local.
• Updated existing tests that asserted the old 3.12 fallback to assert local-match (via monkeypatch of sys.version_info) or an explicit override.
• build.py::_python_version_source: unit tests for override / declared / local-match provenance strings.
• make quality-check green (2580 passed; the 2 TestVersionFlag failures are pre-existing on main — ANSI-color assertion sensitivity, unrelated to this change).

Closes the useful part of #330.

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[promptless]

Promptless prepared a documentation update related to this change.

Triggered by PR #351 (feat(build): match local Python version by default)

This PR's other user-facing behavior (local-interpreter default, 3.10–3.13 supported range, resolution order, and the unsupported-interpreter error) is already covered in the published docs. The one net-new surface is the build-time provenance output — flash build now prints the resolved Python version and its source (matched local interpreter / --python-version override / declared on resource <name>). This suggestion documents that.

Note: since PR #351 is still open, the exact output strings may still shift before merge.

Review: Document flash build resolved Python version provenance output