feat: parsec agent — dedicated non-interactive mode for AI agents

[erishforG]

Vision (v1.0: AI agent Git interface)

A dedicated execution mode for AI agents — no interactive prompts, all output structured JSON.

Proposal

parsec agent start TICKET --title \"Fix auth\"   # non-interactive, JSON only
parsec agent ship TICKET                         # no prompts
parsec agent status                              # full state JSON dump

Features

• All interactive prompts auto-skipped (dialoguer disabled)
• Output is always JSON (implicit --json)
• Structured error codes + hints on failure
• Activate via `PARSEC_AGENT=1` environment variable
• Embeds `doctor --ai` rules for agent self-correction

Supersedes #205 (headless mode).

Milestone

v1.0