Add Exa as a built-in MCP server

[TheGreatAxios]

Summary

• Ships Exa as a default-on built-in MCP server without plugin installation or URL entry.
• Routes canonical web_fetch through Exa MCP by default while hiding the duplicate raw Exa fetch tool.
• Scopes HTTP MCP OAuth credentials to exact server endpoints and documents disable/custom override behavior.

Verification

• bun run check passes
• Focused config, alias, trust, MCP auth, and OAuth isolation tests pass
• Regression tests fail when the Exa default, canonical fetch alias, duplicate hiding, or endpoint credential isolation is removed

Fixes CL-7091

[linear-code]

CL-7091

[TheGreatAxios]

Primary · Comment

Exa is available as an opt-in built-in MCP preset while unconfigured sessions remain inert and HTTP MCP OAuth state stays endpoint-scoped.

No findings.

[TheGreatAxios]

Critic · Comment

The preset activates only through explicit configuration, avoids duplicate startup tools, and preserves MCP credential boundaries across endpoint changes.

No findings.

[TheGreatAxios]

Greybeard · Comment

Configuration resolution owns the built-in preset, while the auth store scopes credentials to normalized endpoint identity.

No findings.

[TheGreatAxios]

Primary · Comment

Exa is the default built-in MCP server, and canonical web_fetch routes through Exa while duplicate raw fetch exposure stays hidden.

No findings.

[TheGreatAxios]

Critic · Comment

The branch keeps Exa default-on, maps canonical web_fetch to Exa web_fetch_exa, and preserves endpoint-scoped MCP OAuth boundaries.

No findings.

[TheGreatAxios]

Greybeard · Comment

Configuration resolution owns default Exa selection, canonical web_fetch owns the alias boundary, and MCP auth state remains scoped to endpoint identity.

No findings.