fix(service): avoid background throttling on macOS

[sunlight798]

Which issue or RFC does this PR close?

Closes #1571.

Rationale for this change

The macOS native service intermittently exceeds its 30-second startup deadline. Failure snapshots show a running LaunchAgent with no listener, while process samples show Python imports and filesystem I/O still in progress.

The evidence points to resource throttling from ProcessType=Background. The personal Server handles user requests and should use standard scheduling.

What changes are included in this PR?

• Use ProcessType=Standard for macOS LaunchAgents.
• Continue recognizing intact older Background definitions so they can be upgraded safely.
• Reconcile native configuration changes even when service metadata is unchanged, while preserving repeated-install idempotency.
• Add regression coverage for legacy compatibility, configuration reconciliation, and native upgrade/restart behavior.

Are there any user-facing changes?

macOS personal services use standard scheduling. Running powercontext service install upgrades an existing Background definition and reloads the service.

The 30-second startup timeout remains unchanged. No public API or persisted metadata format changes.

How was this change tested?

• make check — passed.
• uv run --locked prek run -a — passed.
• Related service, bootstrap, environment, and diagnostic tests — 128 passed, 9 skipped.
• Native lifecycle suite on macOS:
  • Source checkout with Python 3.13 — 6 passed, 1 skipped.
  • Built wheel in a fresh Python 3.12 environment — 6 passed, 1 skipped.
• Local scheduling comparison: Background startup took 8.69/9.70 seconds; Standard took 2.68/1.64 seconds.

Validation was performed locally. CI for this change has not run yet.

AI usage statement

OpenAI Codex assisted

[PsiACE]

totally lgtm