feat: add GitHub Copilot CLI adapter

[ScottRBK]

Summary

• Add CopilotCLIAdapter implementing the AgentAdapter protocol for GitHub Copilot CLI
• Maps Copilot CLI NDJSON events (reasoning_delta, message_delta, result, etc.) to shared StreamEvent types
• Full test coverage: 148 unit+integration tests + 10 E2E tests (174 total, all passing)

Key design decisions

• Reasoning events require --enable-reasoning-summaries CLI flag; disabled by default when include_thinking=False
• Cost defaults to 0.0 (Copilot doesn't expose pricing data)
• Session ID extracted from the result event (not turn_start — a behavioral difference from Claude Code/OpenCode)
• Auto-approve uses --allow-all-tools by default; individual --allow-tool flags when allowed_tools is specified
• Duck-typed Protocol satisfaction (no inheritance required)

Files

File	Purpose
src/agent_shell/adapters/copilot_cli_adapter.py	Adapter implementation (~194 lines)
tests/unit/copilot_fixtures.py	NDJSON event fixtures
tests/unit/test_copilot_cli_*.py	Unit tests (parse, stream, execute, cancel)
tests/integration/test_copilot_cli_integration.py	Integration tests through AgentShell
tests/e2e/test_copilot_cli_e2e.py	E2E tests against real CLI
src/agent_shell/shell.py	Wired adapter into _resolve_adapter()
README.md	Updated with Copilot CLI example

[ScottRBK]

Review

Overall: solid adapter, cleanly mirrors OpenCodeAdapter, good test coverage (unit + integration + E2E). One behavioural item worth addressing before merge, plus minor notes.

assistant.turn_start emits a payload-less system event

copilot_cli_adapter.py:195-196:

if t == "assistant.turn_start":
    events.append(StreamEvent(type="system", content=""))

Copilot's turn_start payload has no sessionId (only result does — see copilot_fixtures.py:893), so this yields StreamEvent(type="system", content="", session_id=None) — zero information.

There's a direct precedent in claude_code_adapter.py:140-143 which guards emission on session_id:

if t == "system":
    if session_id:
        events.append(StreamEvent(type="system", content="", session_id=session_id))

Across the three adapters, type="system" is implicitly a session-id carrier, not a lifecycle marker. An empty one is invisible to execute()'s next((e.session_id for e in chunks if e.session_id), None) harvest and misleading to consumers filtering by type.

Suggestion: drop the turn_start branch entirely. The result branch already emits session_id correctly.

duration isn't surfaced on AgentResponse

README says duration is populated from usage.totalApiDurationMs, but AgentResponse has no duration field — only the StreamEvent carries it, so execute() callers can't see it. Either add it to AgentResponse (cross-adapter change) or tighten the README wording.

Minor

• test_copilot_cli_cancel.py doesn't assert unregister_process_group was called.