User guides: evidence sharing and engagement MCP

Update the three existing report, service API and investigation guides to document the deployed P15/P17 interface. Describe explicit consent and actions, link lifetime/preparation/revocation and no automatic messages; client-wide read-only MCP scope, zero credit budget, one-time secrets, configuration placeholders and Desktop limitations. Do not document P13/P19 as deployed. Read the corresponding published source contracts and interface, retain existing content and use local relative links. Validate types, full Docusaurus build and section anchors. Require independent conformity and root quality review before staging publication.

Worktree creation was blocked by automatic usage review. This reversible documentation edit uses the existing clean docs checkout; no permission/protection is bypassed and no git history or external state is modified by drafting.