fix(docs): correct Base skill install command

[burak33bb]

Updates the Builder Codes agent guide and AI index entries to use the canonical Base MCP skill install command.

Checks:

• node scripts/lint-mdx.js docs/base-chain/specs/reference/builder-codes/agent-developers.mdx
• node --check scripts/agents.js
• node --check scripts/llms.js

[cb-heimdall]

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[burak33bb]

Rebased this onto the latest master and re-verified the canonical Base MCP skill install command across the guide and generated AI index sources. Local checks pass. Ready for review when you get a chance.

[burak33bb]

This is now rebased onto the latest master and mergeable. The change only updates the canonical Base MCP skill install command across the Builder Codes agent guide and generated AI index sources, and the local checks pass.

@youssefea @stephancill — when you get a chance, could you take a quick look? Thanks!