feat(copilot): add first-class session detection, resume, model/allow-all config

[toughhou]

Summary

Adds first-class GitHub Copilot CLI integration with session detection, resume support, and model/allow-all configuration — upgrading copilot from pattern-only support to full session-aware lifecycle management.

Motivation

Currently, agent-deck's copilot support is limited to basic tmux patterns for busy/idle detection. The Copilot CLI (copilot v1.0+) supports --resume, --model, --allow-all, and writes session state to ~/.copilot/session-state/<session-id>/events.jsonl. This PR enables agent-deck to:

1. Detect copilot sessions from disk (scanning events.jsonl for session.start events matching the working directory)
2. Resume sessions automatically using --resume <session-id> when conversation data exists
3. Configure model and allow-all via instance fields or ~/.agent-deck.toml config
4. Propagate session IDs through tmux environment variables for cross-pane awareness

Architecture

Follows the same file-based async detection pattern used by Codex integration:

• copilot_hooks.go: Core implementation — session detection via ~/.copilot/session-state/ scanning, command builder, async goroutine detector
• Respects COPILOT_CONFIG_DIR env override for non-standard installations
• Heuristic conversation detection (events.jsonl > 8KB indicates meaningful conversation)
• Graceful degradation: if no session found, copilot starts fresh

Changes

New Files

• internal/session/copilot_hooks.go — Session detection, command building, async detection
• internal/session/copilot_session_test.go — Comprehensive tests (disk detection, event parsing, command building)
• docs/feature-copilot/copilot-session-issues-and-improvements.md — Design document

Modified Files

• internal/session/instance.go — Added CopilotSessionID/Model/AllowAll fields, wired into Start(), PostStartSync(), SyncSessionIDs*(), clearSessionBinding()
• internal/session/tooloptions.go — Extended CopilotOptions with Model/AllowAll, updated ToArgs()
• internal/session/userconfig.go — Extended CopilotSettings with DefaultModel/AllowAll (TOML)
• internal/session/copilot_test.go — Updated existing tests for new config fields
• internal/tmux/patterns.go — Enhanced copilot busy patterns (Thinking, Generating, Reading, Searching, Running, braille spinner) and prompt patterns

Config Example

[copilot]
default_model = "claude-sonnet-4"
allow_all = true

Testing

• All 28 copilot-related tests pass (go test ./internal/session/ -run Copilot)
• All tmux pattern tests pass (go test ./internal/tmux/ -run Pattern)
• Full build passes (go build ./...)

Closes #556