Add local macOS audio recording

[szymoniwacz]

Summary

• add meetxt record <output.wav> as a separate macOS recording flow
• orchestrate local FFmpeg/AVFoundation capture with Enter-to-stop behavior and exclusive final publication
• add CLI and recorder coverage plus setup, usage, scope, requirement, and decision documentation

Scope check

The implementation stays within issue #8: default-device WAV recording only. It does not add device selection, combined record/transcribe, Linux support, integrations, diarization, summaries, or cloud storage.

Validation

• bundle exec rake — 47 examples pass; RuboCop reports no offenses
• gem build meetxt.gemspec — succeeds
• ./scripts/check-workflow-leak.sh — clean
• git diff --check — clean

A real microphone capture was not automated because it requires macOS hardware permission and interactive input. Automated tests cover command construction, native stop signaling, output publication, collisions, failed/empty capture cleanup, unsupported platforms, and missing FFmpeg.

Diff risk

Diff-risk: Medium
Signals: Architecture boundary / external-process change, cross-layer code + docs change
Evidence: 12 files add one recorder boundary, CLI dispatch, focused tests, and matching product documentation; no dependency, migration, CI, auth, or secret changes
Reviewer focus: FFmpeg AVFoundation command, Enter-to-stop process lifecycle, and exclusive temporary-to-final WAV publication
Required action: Manually smoke-test meetxt record meeting.wav on macOS with microphone permission before release

Dependencies and follow-ups

• Runtime recording requires user-installed FFmpeg on PATH
• Configurable device selection remains deferred

Closes #8