feat(mcp): support VS Code-style variable interpolation in MCP server config

[Onkar-Mane]

Adds support for ${workspaceFolder}, ${workspaceFolderBasename}, ${userHome}, and ${env:VAR} in MCP server command/args/cwd/env, resolved once right before the stdio process is spawned.

Summary

• New resolveMcpVariables utility (core/util/resolveMcpVariables.ts) recursively resolves the variables in strings/arrays/objects, leaving unknown variables and ${{ secrets.X }} untouched.
• Wired into MCPConnection.constructStdioTransport, the single choke point for all stdio MCP servers regardless of config source (YAML or JSON).
• Fixed resolveCwd's absolute-path check to use path.isAbsolute instead of a Unix-only charAt(0) === "/" check, needed for ${workspaceFolder}-derived Windows paths to be recognized correctly.
• ${workspaceFolder} intentionally stays unresolved (rather than falling back to homedir) for remote workspaces, to avoid silently pointing configs at the wrong directory.

Test plan

• core/util/resolveMcpVariables.test.ts - 18 unit tests
• core/context/mcp/MCPConnection.vitest.ts - new wiring/integration tests
• tsc --noEmit clean
• Manually verified in the Extension Development Host with a .continue/mcpServers/*.yaml using ${workspaceFolder} in cwd/args/env

[github-actions]

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