why

The v3 MCP docs now need to describe the hosted server behavior where start can attach to an existing Browserbase session via an optional sessionId.

what changed

• Updated the MCP setup and tools docs to document optional start.sessionId.
• Clarified that omitting sessionId creates a new Browserbase session.
• Added a patch changeset for @browserbasehq/stagehand-docs.

test plan

• pnpm exec prettier --check packages/docs/v3/integrations/mcp/setup.mdx packages/docs/v3/integrations/mcp/tools.mdx

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Summary by cubic

Added docs for optional sessionId on MCP start to attach to an existing Browserbase session; omitting it creates a new session. Addresses STG-1837 and publishes a patch to @browserbasehq/stagehand-docs.