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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* 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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* 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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* 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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* fix: lint error in cleanBeadsRuntimeFiles

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

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* 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>

internal/config/types.go

@@ -38,7 +38,7 @@ type TownSettings struct {
 	Version int    `json:"version"` // schema version
 
 	// DefaultAgent is the name of the agent preset to use by default.
-	// Can be a built-in preset ("claude", "gemini", "codex")
+	// Can be a built-in preset ("claude", "gemini", "codex", "cursor", "auggie", "amp")
 	// or a custom agent name defined in settings/agents.json.
 	// Default: "claude"
 	DefaultAgent string `json:"default_agent,omitempty"`
@@ -190,7 +190,7 @@ type RigSettings struct {
 	Runtime    *RuntimeConfig    `json:"runtime,omitempty"`     // LLM runtime settings (deprecated: use Agent)
 
 	// Agent selects which agent preset to use for this rig.
-	// Can be a built-in preset ("claude", "gemini", "codex")
+	// Can be a built-in preset ("claude", "gemini", "codex", "cursor", "auggie", "amp")
 	// or a custom agent defined in settings/agents.json.
 	// If empty, uses the town's default_agent setting.
 	// Takes precedence over Runtime if both are set.