feat(phase-1): foundation contract — schema, exit codes, CI

[sonpiaz]

Summary

Implements Phase 1 of the defend-the-niche plan: the versioned output schema, exit code conventions, and a CI matrix. Sub-spec docs landed in the preceding commit on this branch.

Locked pitch (paste verbatim for grep-confirmation):

Watch any social video → get an architecture diagram, working component, runnable notebook, or step-by-step cheat sheet — automatically.

Items shipped

Item	Where
Sub-spec docs (output schema, exit codes)	docs/output-schema.md, docs/exit-codes.md
WATCH_OUTPUT_VERSION header + EXIT trailer	bin/watch (text mode)
--format json compact JSON	bin/watch (json mode)
Structured exit codes across all bin/*	bin/{watch,dl-video,extract-frames,transcribe,audio-q,models}
POSIX stat fix	bin/transcribe, bin/audio-q
CI matrix (macOS + Ubuntu)	.github/workflows/ci.yml
Schema smoke test	tests/test-output-schema.sh
CI badge	README.md

Test plan

• bash -n on all 6 bin scripts — pass
• watch --help — exit 0
• watch --version — exit 0
• watch with no args — exit 64
• bash tests/test-output-schema.sh — pass
• CI green on the matrix once pushed

Reviewer checklist

• Sub-spec docs read top to bottom
• JSON output validates against the schema for at least one example
• Exit codes match the table in docs/exit-codes.md
• No new dependencies beyond what install.sh already required
• README CI badge renders