Add gh-style --json CLI mode for LLM agents

[rafaeelricco]

Summary

Adds machine-readable --json output for commit generate and commit doctor, following the same pattern as gh. When --json is set, the CLI skips Clack prompts and writes structured JSON to stdout; errors go to stderr as JSON.

Automation flags (require --json):

• --adjust <text> — refine the message once
• --dry-run — generate only, no git writes
• --commit — commit with the generated message
• --push — push after commit
• --yes — publish branch or force-with-lease without confirm

Interactive commit / commit generate behavior is unchanged.

Changes

• New src/cli/json-io.ts — JSON schemas, CliError codes, stdout/stderr writers
• New src/cli/generate-flags.ts — GenerateRunMode union and flag parsing
• Updated src/cli/parser.ts — flag-aware command parsing and help text
• Updated src/cli/commit.ts — run(mode) with runJson / pushJson paths
• Updated src/cli/doctor.ts — doctor --json with checks array
• Updated index.ts — passes command.mode / command.json (thin switch unchanged)
• Docs — AGENTS.md + README agent section
• Tests — parser, generate-json, doctor-json coverage

Example usage

commit generate --json | jq -r '.message'
commit generate --json --commit
commit generate --json --commit --push
commit doctor --json | jq '.ready'

Verification

• pnpm run typecheck
• pnpm run lint:ci
• pnpm test (78 passed)
• pnpm run build