feat: add cursor agent support

[bobobobo]

PR Summary: Add Cursor agent support

Overview

Adds Cursor CLI as a fourth agent backend for Chief. Users can run PRD loops with Cursor's agent CLI in addition to Claude Code, Codex, and OpenCode.

What's included

Agent integration

• internal/agent/cursor.go — New CursorProvider implementing loop.Provider:
  • Loop command invokes agent -p --output-format stream-json --force --workspace <dir> --trust with prompt on stdin
  • Interactive, convert, and fix-JSON commands wired for Cursor
  • CleanOutput handles both single JSON and NDJSON (finds last result/success)
  • Log file: cursor.log
• internal/agent/resolve.go — Added "cursor" case to Resolve(); resolution order unchanged (flag → CHIEF_AGENT → config → default). Error message updated to list all four providers.

Output parsing

• internal/loop/cursor_parser.go — Parser for Cursor CLI stream-json NDJSON:
  • Maps system/init → iteration start, assistant → assistant text / story status / <chief-complete/>, tool_call → tool start/result
  • Supports all Cursor tool types: read, write, edit, shell, grep, glob, ls, delete, web fetch, web search, function (with display names and result summaries for the TUI)

Configuration & CLI

• internal/config/config.go — AgentConfig comment updated to include Cursor.
• cmd/chief/main.go — --agent accepts cursor; help and error strings updated to mention all four agents.

Tests

• internal/agent/cursor_test.go — Tests for CursorProvider (commands, CleanOutput, ParseLine).
• internal/agent/resolve_test.go — Tests for resolving cursor (and path/config).
• internal/loop/cursor_parser_test.go — Tests for Cursor NDJSON parsing (events and tool calls).

Documentation

• Installation — New "Option D: Cursor CLI" (install, PATH/agent.cliPath, agent login, chief --agent cursor / config).
• Quick start — Cursor CLI in prerequisites and agent --version in verify step.
• Configuration — agent.provider and flags table include cursor; Cursor auth and headless/trust/force usage noted.
• Troubleshooting — Cursor in "Agent not found", log file names, and "Permission denied" / "Stuck" log references.
• PRD format — Log file list includes cursor.log.
• AgentSupport.vue — Cursor CLI added to the agents list.

How to use

• chief --agent cursor or CHIEF_AGENT=cursor or agent.provider: cursor in .chief/config.yaml
• Optional agent.cliPath (or CHIEF_AGENT_PATH) if agent isn't on PATH
• Cursor is run headless with --trust and --force so Chief can drive it without interactive prompts.

Testing

• New and updated unit tests for Cursor provider, resolver, and cursor parser; existing agent flow unchanged.

[MiniCodeMonkey]

@bobobobo This is amazing. Thanks for adding this PR! I will review and get this merged in!