feat: add cursor agent support

[MiniCodeMonkey]

Summary

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.

Based on #48 by @bobobobo — rebased onto current main (post prd.json removal) with the following adjustments:

• Removed ConvertCommand and FixJSONCommand (no longer in the Provider interface)
• Replaced EventStoryStarted/extractStoryID/ralph-status with EventStoryDone/<chief-done/> to match current parser patterns
• Extracted duplicate anonymous struct in CleanOutput to a named cursorResultLine type
• All doc changes applied against current markdown-only content

What's included

Agent integration

• internal/agent/cursor.go — CursorProvider implementing loop.Provider with --output-format stream-json, --force, --trust flags
• internal/agent/resolve.go — Added "cursor" case to Resolve()

Output parsing

• internal/loop/cursor_parser.go — Parser for Cursor CLI stream-json NDJSON, mapping all Cursor tool types to Chief events

Configuration & CLI

• cmd/chief/main.go — --agent accepts cursor; help text updated
• internal/config/config.go — AgentConfig comment updated

Tests

• internal/agent/cursor_test.go, internal/agent/resolve_test.go, internal/loop/cursor_parser_test.go

Documentation

• Installation, quick start, configuration, troubleshooting, PRD format, and AgentSupport.vue all updated

How to use

chief --agent cursor
# or: CHIEF_AGENT=cursor
# or: agent.provider: cursor in .chief/config.yaml

Test plan

• go build ./... succeeds
• go test ./... — all 9 packages pass
• Manual test with Cursor CLI installed