feat: Add Cursor, Auggie, and Sourcegraph AMP agent presets (#247)

* feat: add Cursor Agent as compatible agent for Gas Town

Add AgentCursor preset with ProcessNames field for multi-agent detection:
- AgentCursor preset: cursor-agent -p -f (headless + force mode)
- ProcessNames field on AgentPresetInfo for agent detection
- IsAgentRunning(session, processNames) in tmux package
- GetProcessNames(agentName) helper function

Closes: ga-vwr

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* refactor: centralize agent preset list in config.go

Replace hardcoded ["claude", "gemini", "codex"] arrays with calls to
config.ListAgentPresets() to dynamically include all registered agents.

This fixes cursor agent not appearing in `gt config agent list` and
ensures new agent presets are automatically included everywhere.

Also updated doc comments to include "cursor" in example lists.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* test: add comprehensive agent client tests

Add tests for agent detection and command generation:

- TestIsAgentRunning: validates process name detection for all agents
  (claude/node, gemini, codex, cursor-agent)
- TestIsAgentRunning_NonexistentSession: edge case handling
- TestIsClaudeRunning: backwards compatibility wrapper
- TestListAgentPresetsMatchesConstants: ensures ListAgentPresets()
  returns all AgentPreset constants
- TestAgentCommandGeneration: validates full command line generation
  for all supported agents

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* feat: add Auggie agent, fix Cursor interactive mode

Add Auggie CLI as supported agent:
- Command: auggie
- Args: --allow-indexing
- Supports session resume via --resume flag

Fix Cursor agent configuration:
- Remove -p flag (requires prompt, breaks interactive mode)
- Clear SessionIDEnv (cursor uses --resume with chatId directly)
- Keep -f flag for force/YOLO mode

Updated all test cases for both agents.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* feat(agents): add Sourcegraph AMP as agent preset

Add AgentAmp constant and builtinPresets entry for Sourcegraph AMP CLI.

Configuration:
- Command: amp
- Args: --dangerously-allow-all --no-ide
- ResumeStyle: subcommand (amp threads continue <threadId>)
- ProcessNames: amp

Closes: ga-guq

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix: lint error in cleanBeadsRuntimeFiles

Change function to not return error (was always nil).

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix: beads v0.46.0 compatibility and test fixes

- Add custom types config (agent,role,rig,convoy,event) after bd init calls
- Fix tmux_test.go to use variadic IsAgentRunning signature

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* docs: update agent documentation for new presets

- README.md: Update agent examples to show cursor/auggie, add built-in presets list
- docs/reference.md: Add cursor, auggie, amp to built-in agents list
- CHANGELOG.md: Add entry for new agent presets under [Unreleased]

Addresses PR #247 review feedback.

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---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

docs/reference.md

@@ -359,7 +359,7 @@ gt config agent remove <name>     # Remove custom agent (built-ins protected)
 gt config default-agent [name]    # Get or set town default agent
 ```
 
-**Built-in agents**: `claude`, `gemini`, `codex`
+**Built-in agents**: `claude`, `gemini`, `codex`, `cursor`, `auggie`, `amp`
 
 **Custom agents**: Define per-town in `mayor/town.json`:
 ```bash