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

* 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>

README.md

@@ -274,12 +274,14 @@ gt crew add <name> --rig <rig>  # Create crew workspace
 ```bash
 gt agents                   # List active agents
 gt sling <issue> <rig>      # Assign work to agent
-gt sling <issue> <rig> --agent codex    # Override runtime for this sling/spawn
+gt sling <issue> <rig> --agent cursor   # Override runtime for this sling/spawn
 gt mayor attach             # Start Mayor session
-gt mayor start --agent gemini           # Run Mayor with a specific agent alias
+gt mayor start --agent auggie           # Run Mayor with a specific agent alias
 gt prime                    # Alternative to mayor attach
 ```
 
+**Built-in agent presets**: `claude`, `gemini`, `codex`, `cursor`, `auggie`, `amp`
+
 ### Convoy (Work Tracking)
 
 ```bash