feat(cli): add mcp subcommand for managing MCP servers

[christso]

Closes #346

Summary

Adds an allagents mcp subcommand surface with five operations for managing MCP servers independently of full plugin installs. Mirrors the claude mcp command surface and persists servers in a new top-level mcpServers: field in workspace.yaml (parallel to mcpProxy:).

New commands

Command	Description
allagents mcp add <name> <commandOrUrl>	Add a server to workspace.yaml and sync to all configured clients
allagents mcp remove <name>	Remove a server from workspace.yaml and unsync from clients
allagents mcp list	List MCP servers defined in workspace.yaml
allagents mcp get <name>	Show the workspace definition for a server
allagents mcp update	Sync MCP servers only, without touching other artifacts

mcp add supports:

• --transport http|stdio (auto-detected from URL if omitted)
• --arg <value> (repeatable, stdio only)
• -e/--env KEY=VALUE (repeatable, stdio only)
• --header KEY=VALUE (repeatable, http only)
• --client <csv> — limit which clients receive the server
• --force / -f — replace an existing server with the same name (used for updates)

Architecture

• src/models/workspace-config.ts — new McpServerConfigSchema + top-level mcpServers: field.
• src/core/vscode-mcp.ts — collectMcpServers now accepts workspace-level servers and merges them with plugin-supplied servers (workspace overrides plugin, with a warning).
• src/core/mcp-sync.ts — new module containing syncMcpServers (the per-client MCP sync loop, extracted from sync.ts::syncWorkspace) and syncMcpOnly (standalone MCP-only pipeline used by mcp update and by add/remove auto-sync).
• src/core/sync.ts — syncWorkspace now delegates to the shared syncMcpServers. Removes ~100 lines of duplicated per-client MCP blocks.
• src/core/mcp-servers.ts — new module with CRUD helpers for the top-level mcpServers: field, plus buildMcpServerConfigFromFlags for translating CLI flags into an McpServerConfig.
• src/cli/commands/mcp.ts / src/cli/metadata/mcp.ts — new CLI commands with agent-help metadata.

Ownership model

Honors the existing MCP ownership rule per CLAUDE.md: only servers AllAgents added are tracked via trackedServers in sync state and can be modified or removed. Pre-existing user-managed servers in .mcp.json / .copilot/mcp-config.json are preserved untouched (verified in E2E).

Tests

• 20 new unit tests in tests/unit/core/mcp-servers.test.ts covering add/remove/get/list, config building from flags, and KEY=VALUE parsing.
• Full suite: 1178 pass, 4 skip, 0 fail.

Test plan

• bun test — all tests pass
• bunx tsc --noEmit — clean
• bun run lint — clean
• Red E2E: confirmed baseline (mcp subcommand did not exist on main)
• Green E2E: ran in /tmp/mcp-test-e2e with claude,copilot clients
  • mcp add deepwiki <https-url> → writes to workspace.yaml, syncs to .mcp.json and .copilot/mcp-config.json
  • mcp add gh-server npx --arg=-y --arg=@modelcontextprotocol/server-github -e GH_TOKEN=... → stdio with args and env
  • mcp add deepwiki ... --force --header Authorization=Bearer-token → replaces existing server (update workflow)
  • mcp list → shows both servers with transport details
  • mcp get deepwiki → prints YAML dump
  • mcp add claude-only ... --client claude → synced only to .mcp.json, not to copilot
  • mcp remove gh-server → removes from workspace.yaml and both client configs
  • mcp update → sync-only, shows "No MCP server changes." when nothing changed
  • Duplicate add without --force → exit 1 with clear error
  • mcp add bad https://... --transport stdio → exit 1 ("stdio transport requires a command, not a URL")
  • mcp get nonexistent → exit 1 with clear error
  • Ownership: pre-existing user-managed server in .mcp.json survived mcp remove of a different server

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