Agent abstraction Phase A: introduce CodingAgent protocol (behavior-preserving)

[dhilgaertner]

Summary

Phase A of the agent-abstraction work scoped by #150. Introduce a CodingAgent protocol and route all Claude Code call sites through it — zero user-visible change, zero new features. Lays the foundation for Phases B and C.

Context

Full design in docs/agent-abstraction-spec.md §12 Phase A. Non-trivially informed by Codex CLI research and an audit of ~435 claude references across the codebase.

The existing Packages/CrowClaude/ is already a Claude-specific Swift package containing ClaudeLauncher. It's the natural home for ClaudeCodeAgent.

Checklist

• Define CodingAgent, AgentKind, AgentRegistry, StateSignalSource, AgentStateTransition, HookConfigWriter protocols in CrowCore (or a new CrowAgent package)
• Implement ClaudeCodeAgent in Packages/CrowClaude/; register it in AgentRegistry
• Rename ClaudeState → AgentActivityState; .claudeLaunched → .agentLaunched; onLaunchClaude → onLaunchAgent
• Update TerminalReadinessTests for the rename
• Move HookConfigGenerator from Sources/Crow/App/ into Packages/CrowClaude/ as ClaudeHookConfigWriter
• Move the hook-event state-machine logic out of AppDelegate.hook-event into a ClaudeHookSignalSource in CrowClaude; AppDelegate consumes AgentStateTransition values
• Wrap ClaudeLauncher.generatePrompt/launchCommand behind ClaudeCodeAgent
• swift test passes across every package; launching a session looks identical; hook state transitions unchanged

Non-goals

• No Codex code
• No user-visible UI change
• No Session model change (that's Phase B)

Parent: #150

[dhilgaertner]

Blocks: #167, #168, #169 (Phase B, C, D).