Add fx MCP install target

[masnwilliams]

Summary

• add fx as a kernel mcp install target
• write Kernel's Streamable HTTP and OAuth configuration to ~/.fx/mcp.json
• preserve existing fx MCP servers and secure the profile directory and file
• print fx-specific authentication and verification steps

Testing

• make test
• built the CLI and smoke-tested kernel mcp install --target fx with a clean home directory
• verified generated permissions are 0700 for ~/.fx and 0600 for mcp.json
• git diff --check

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Note

Low Risk
Local CLI config-file writes only; no auth, network, or shared-service changes. Restrictive file modes slightly reduce risk of leaking MCP config.

Overview
Adds fx as a kernel mcp install target so Kernel can be registered as an HTTP MCP server with OAuth in ~/.fx/mcp.json.

Install merges a kernel entry into the existing mcp map without dropping other servers, writes the file with 0600 (and 0700 for a newly created ~/.fx), and prints fx-specific reload/auth/verify steps. Tests cover merge behavior and permission handling.

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[rgarcia]

reviewed — looks good, approving. one suggestion worth considering before/after merge:

Suggestions

• cmd/mcp/mcp.go:248-250 — the unconditional Chmod(dir, 0700) repermissions a pre-existing ~/.fx that holds unrelated profile state (and on non-macOS, the credential store). Security value is near-zero once the file itself is 0600 — a world-readable dir only leaks filenames. Consider dropping it so existing dirs keep their permissions; MkdirAll(dir, 0700) already covers the create-new case.

Nits

• cmd/mcp/mcp_test.go:17 / cmd/mcp/mcp_test.go:82-87 — the test pre-creates the dir at 0755 and then asserts 0700, which currently requires the unconditional chmod above. Update alongside any change to writePrivateJSONFile.
• cmd/mcp/mcp_test.go — no coverage for the clean-install path (no pre-existing file/dir); a second small case would lock in create-at-0700/0600 behavior.

Follow-up (not this PR)

• other targets' configs also hold tokens (~/.claude.json especially) — worth a follow-up ticket to apply private-perm writes there

Also verified the fx integration against fx's published MCP docs (fx.sh/docs/capabilities/mcp): config path/schema, "type": "http" + url, empty oauth object selecting delegated user authorization, and all three /mcp commands in the post-install copy match exactly.