Preserve scheduled meeting prompt context

[r3dbars]

Why

Follow-up to PR #1119 after pre-merge review found the browser mic prompt edge needed a safer handoff with scheduled calendar prompts.

What changed

• Native mic candidates still prefer matching calendar context.
• Generic browser mic candidates stay neutral for fresh prompts.
• If a scheduled calendar prompt is already pending or snoozed, browser mic activity cannot bypass it with a second generic prompt.
• Added regression coverage for the pending scheduled Meet case.

Proof

• codex-review against PR Detect spontaneous calls via mic activity and offer to record #1119 branch found the original P2 issue.
• bash run-tests.sh --filter MeetingPromptDetector passed, 39 of 39.
• bash build-deps.sh --force passed.
• bash build.sh --no-open passed.
• bash run-tests.sh passed, 5342 of 5342.
• bash run-integration-smoke.sh passed.
• final codex-review --mode local was clean with no accepted/actionable findings.