feat: unified MCP server configuration API

[ScottRBK]

Summary

• Adds add_mcp_server / remove_mcp_server / list_mcp_servers to AgentShell and the AgentAdapter protocol so callers stop hand-coding per-agent dispatch when wiring MCP servers (e.g. forgetful for evals).
• Implements all three methods for Claude Code, OpenCode, and Copilot CLI. All adapters write user-scope config so registrations persist across execute/stream calls.
• MCPServerSpec dataclass validates STDIO/HTTP exactly-one-of fields at construction time.

Per-adapter behaviour

Agent	Mechanism	Storage
Claude Code	claude mcp add/remove --scope user subprocess	~/.claude.json (managed by CLI)
OpenCode	direct JSON write	~/.config/opencode/opencode.json
Copilot CLI	direct JSON write	~/.copilot/mcp-config.json

list_mcp_servers raises NotImplementedError for Claude Code — tracked in #3.

Semantics

• Add is idempotent: existing entry with the same name is overwritten (Claude Code does pre-remove + add; file-based adapters overwrite the JSON key).
• Remove warns rather than raises if the named server is not found (UserWarning).

Test plan

• Unit: MCPServerSpec validation (9 tests covering all invalid combinations)
• Unit: AgentShell passthrough delegation + exception propagation
• Integration (Claude Code): mocked subprocess asserts claude mcp add/remove argv shape, -e/--header translation, error propagation, overwrite-via-pre-remove
• Integration (OpenCode): real file I/O against monkeypatch HOME=tmp_path; preserve unrelated config keys, overwrite same-name, round-trip via list_mcp_servers, warn on missing remove
• Integration (Copilot CLI): same pattern as OpenCode
• Full CI suite: 192 passed (was 158)

Closes #2

🤖 Generated with Claude Code

[ScottRBK]

I found one issue worth addressing:

list_mcp_servers() can now crash on malformed MCP config entries in both src/agent_shell/adapters/opencode_adapter.py and src/agent_shell/adapters/copilot_cli_adapter.py. The new parsing code builds MCPServerSpec directly from JSON using .get(...), so missing or empty required fields like command or url raise ValueError and abort the entire listing. Since these config files are user-editable, a single bad entry can break the whole method instead of skipping or reporting the malformed server cleanly.

It would be safer to catch ValueError per entry and either skip invalid servers with a warning or raise a more targeted error that identifies the bad entry.