refactor: extract AgentProvider abstraction for multi-provider support

[aterrylu]

Summary

• Extract all Claude Code-specific logic from sessions.ts into providers/claude-code.ts implementing the AgentProvider interface
• Create provider registry (providers/index.ts) with getProvider() / getAllProviders() — sessions.ts delegates to the provider instead of hardcoding CC flags
• Expand AgentProvider interface with resolveBinary(), buildArgs(), buildEnv(), attachStartupWatcher(), and ProviderCapabilities type
• Add PtyHandle abstraction to keep node-pty dependency out of the core package
• Add ResolvedSpawnOptions type for pre-resolved spawn configuration passed to providers

This is Phase 0 of multi-provider support (Codex CLI + Gemini CLI). Pure refactor — zero behavior change, all 70 tests pass.

sessions.ts (before):  ~830 lines, CC flags hardcoded throughout
sessions.ts (after):   ~470 lines, provider-agnostic orchestration
providers/claude-code.ts: ~370 lines, all CC-specific logic
providers/index.ts:    ~40 lines, provider registry

Architecture

sessions.ts (orchestrator)
  │
  ├── validates options, generates IDs
  ├── resolves provider via getProvider(name)
  ├── calls provider.buildArgs() + provider.buildEnv()
  ├── spawns PTY with provider.resolveBinary()
  ├── calls provider.attachStartupWatcher() if configured
  └── manages lifecycle, output buffer, persistence
          │
          ▼
providers/claude-code.ts
  ├── resolveBinary() — finds claude in PATH
  ├── buildArgs() — --session-id, --append-system-prompt, --settings, --mcp-config, etc.
  ├── buildEnv() — AUTONOMOS_* vars, ANTHROPIC_* overrides, PATH setup
  ├── attachStartupWatcher() — auto-dismiss trust/channels prompts
  └── capabilities — 13 hook events, channels messaging, session fork, etc.

Why this matters

sessions.ts was an 830-line monolith with CC-specific flags hardcoded throughout. Adding Codex or Gemini support would have required forking the entire function. Now, adding a new provider means creating a single file that implements AgentProvider — the orchestration layer is shared.

Research backing this: docs/research/multi-provider-support.md (POC validated with both Codex v0.118.0 and Gemini v0.30.0).

Test plan

• make check passes (lint + typecheck + 70/70 tests)
• Rebased on latest main
• Polished by 3 review agents (code-reviewer, code-simplifier, silent-failure-hunter)
• Fixed behavioral regression in attachStartupWatcher expectChannels logic (caught by code-reviewer)
• Fixed redundant capability re-resolution in buildArgs (caught by code-reviewer)
• Improved error logging in PTY write failures and timeout warnings (caught by silent-failure-hunter)

🤖 Generated with Claude Code

[nox-0x]

Clean abstraction — well-structured provider interface, sessions.ts is now cleanly provider-agnostic, and all CC-specific logic is correctly extracted. The attachStartupWatcher improvements (agent labels, ptyDead guard, cleaner console output) are a meaningful upgrade. No blocking issues. LGTM.