refactor: split proxy.ts into 3-module SRP structure

[dean0x]

Problem

src/cli/commands/proxy.ts is 918 lines with multiple responsibilities: relay lifecycle management (spawn/wait/kill), CLI argument handling (enable/disable/status), configuration state management, and TUI integration.

Context

• Surgical extractions already landed (CPLX-2/3/7): spawnRelayAndWaitForPort, resolveProcessState, formatProcessLine, readPidFile
• Full SRP refactor requires splitting into 3-5 focused modules
• Discretionary improvement; code is functional but violates single-responsibility principle
• Ripples to init.ts and agents.ts importers

Proposed Structure

1. proxy-relay.ts — Relay lifecycle (spawn, wait for health, kill, doctor)
2. proxy-config.ts — Configuration state (read/write settings.json, apply routing)
3. proxy-cli.ts — CLI commands (enable/disable/status actions)
4. proxy.ts — Re-export main entry point

Acceptance Criteria

• Each module has one reason to change
• No circular dependencies
• All tests pass (81 regression tests from PR feat: external model routing (devflow proxy) + per-agent model config (devflow agents) #269)
• No breaking changes to importers (cli.ts, init.ts)
• Documentation updated (CLAUDE.md, cli-reference.md)

Related

• PR feat: external model routing (devflow proxy) + per-agent model config (devflow agents) #269 resolution-summary.md, Fix Separately section
• CPLX-2, CPLX-3, CPLX-7 (prior extractions)