feat(contract): add API schema drift detection

[sorlen008]

Closes #50

What this adds

A contract subcommand that detects when upstream APIs change their response shape — the #1 cause of silent adapter breakage.

Commands

• opencli contract snapshot <site> <command> [args...] — run a command and save its response schema as a baseline
• opencli contract check <site> <command> [args...] — run the command again and diff against the baseline
• opencli contract list — list all saved baselines

How it works

1. Schema capture — recursively traverses JSON responses and extracts structure (keys, types, nested objects, array element types). Array schemas merge all elements into a union.
2. Schema diff — compares baseline vs current: fields added (+), removed (-), type changed (~). Recurses into nested objects and array items. Paths use dot notation (data.items[].slug).
3. Storage — baselines saved at ~/.opencli/contracts/<site>/<command>.json
4. Exit codes — check exits with code 1 if drift is detected, making it CI-friendly.

Example

# Save baseline
opencli contract snapshot hackernews top

# Later, check for drift
opencli contract check hackernews top
# ✓ No schema drift detected

# If the API changed:
# ~ data[].score: number → string
# - data[].by
# + data[].author
# ✗ 3 schema differences detected

Files

• src/contract.ts (new) — schema capture, diff engine, storage, formatting
• src/contract.test.ts (new) — 25 unit tests covering all core functions
• src/cli.ts — wired up contract subcommand group

Testing

• npx tsc --noEmit — zero type errors
• npm test — all 25 new tests pass. 3 pre-existing failures unrelated.

[sorlen008]

UAT Results (Windows 11)

Test	Result
Unit tests (25/25)	PASS
Type check (tsc --noEmit)	PASS
Schema capture + diff (manual)	PASS — correctly detects field removal, addition, type changes
Contract storage (save/load round-trip)	PASS
CLI wiring (contract list, --help)	PASS

All 5 tests pass. No bugs found — clean implementation.