feat(cli): restore --json on pick/stats/diff for read-command parity

[fyodoriv]

Why this is needed

The CLI surface had drifted out of consistency: tasks list --json (PR #54) round-tripped through JSON.parse, but tasks pick, tasks stats, and tasks diff lost their --json flag in commit ccf1360 (2026-03-17, "remove unused features"). One read command was JSON-scriptable, three were not.

The justification for tasks list --json documented in packages/cli/README.md:42 ("round-trips through JSON.parse for scripting") applies equally to pick/stats/diff. Scripts and CI integrations expect the four read commands to share one flag shape so they can switch between commands without re-plumbing parsing. Closes cli-restore-json-on-read-commands (P1 hardening from PR #58).

Summary

Restoring --json across all four read commands so any script can choose a command and parse its output with the same JSON.parse(stdout) call.

Command	--json shape
pick --json	{picked: false} when empty, else {picked: true, summary, priority, file, line, metadata, candidates, unblocks} (matches commit a567140)
list --json	Unchanged — array of {id, summary, priority, tags, blocked, claimed, file, line}
stats --json	Full QueueStats object (total, byPriority, blocked, claimed, available, fileCount, throughput, topAgents)
diff --json	Full QueueDiff object (ref, added, removed, claimed, hasChanges)

Tests

Five new test cases in packages/cli/src/cli.test.ts pin the parity contract so the same drift fails CI next time:

• pick --json outputs structured JSON
• pick --json outputs {picked: false} for empty queue
• stats --json outputs structured JSON
• diff --json outputs structured JSON (uses git add -f to bypass any global gitignore that excludes TASKS.md in the test temp dir)
• --help advertises --json for every read command (pick, list, stats, diff) — fails CI if any of the four read commands stops listing `--json`

Test plan

• `npm run build` — clean
• `npm test` — 402 tests pass (was 397, +5 new)
• `npm run lint` — 0 errors
• `npx -y @tasks-md/lint TASKS.md` — 0 errors
• Manual smoke: `tasks pick --help`, `tasks stats --help`, `tasks diff --help` all list `--json`

---

🤖 Written by an agent, not Fyodor. Ping me if this looks off.