docs(mcp): align capability and access guidance with server

[carlomarmo]

Summary

The MCP guide describes a read-only, seven-tool interface, while the server advertises nine tools and supports authenticated thread posting. Align the guide with the existing global and corpus-scoped schemas and handlers.

Changes

• Document list_relationships and create_thread_message on both tool catalogues, including the authenticated endpoint.
• Clarify that posting requires corpus/thread visibility and an unlocked thread, without a separate WRITE permission; correct the identifier description.
• Describe bounded text retrieval, the full-text document resource, passage/block search, and the passage-only substring fallback.
• Replace the outdated architecture diagram with a concise description and link to authoritative code definitions.
• Add a changelog fragment. Runtime behavior is unchanged.

Test plan

• python -m mkdocs build: passes before and after the change, with the same 219 existing warnings and no new warnings.
• Checked both documented tool catalogues against the source definitions: nine tools each, matching names and order.
• Parsed the JSON examples and checked the generated HTML for both nine-row tool tables and the posting-permission clarification.
• python scripts/collate_changelog.py --check and git diff --check: pass.
• Relevant pre-commit hooks: trailing whitespace, end-of-file fixer, and changelog validation.
• Backend/frontend suites were not run for this documentation-only change.

Checklist

• Documentation built and checked against source
• Changelog fragment added
• No runtime, dependency, migration, or frontend changes

[github-actions]

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[carlomarmo]

I have read the CLA Document and I hereby sign the CLA