Issue for this PR

Closes #49983

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When configuring a local MCP server in opencode.jsonc with an env block (the standard key used by Claude Desktop, Cursor, and OpenCode's own LSP config), the environment variables were not being passed to the spawned child process.

Root causes:

1. Local schema in packages/core/src/v1/config/mcp.ts and packages/core/src/config/mcp.ts only accepted environment, causing Effect Schema's decode with onExcessProperty: "ignore" to strip env.
2. ConfigV2Compat.lower in packages/opencode/src/config/v2-compat.ts similarly omitted env from the intermediate Server schema.
3. connectLocal in packages/opencode/src/mcp/index.ts only spread ...mcp.environment when constructing StdioClientTransport.

Fix:

• Added env as an optional string record in V1 and V2 Local MCP schemas and Server in v2-compat.ts.
• Forwarded env in migrateMcp.
• Merged both ...mcp.environment and ...mcp.env into the child process environment in connectLocal.

How did you verify your code works?

• Added unit tests in packages/opencode/test/config/v2-compat.test.ts verifying env preservation in both flat and enveloped MCP configs.
• Added an end-to-end unit test in packages/opencode/test/mcp/lifecycle.test.ts verifying that TEST_MCP_ENV_VAR configured via env is received by the spawned child process MCP server.
• Ran test suites:
  • bun test test/config/v2-compat.test.ts (37/37 pass)
  • bun test test/mcp/lifecycle.test.ts (22/22 pass)
  • bun test test/mcp (62/62 pass across 10 files)
  • bun test packages/core/test/config/config.test.ts (15/15 pass)
• Ran oxlint with 0 errors.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR