docs: OAB MCP Facade operator guide

[chaodu-agent]

Docs-only. Adds docs/oab-mcp-facade.md covering the facade shipped in #1448: how to enable it (broker [mcp] section or openab-agent mcp-facade), the search_capabilities/execute_capability surface, audit logging, client registration examples (including the kiro --agent per-agent config gotcha found during b2 deployment), the loopback trust model, and pod-vs-fleet positioning vs octobroker per ADR §4.1. Cross-links docs/gmail-native.md (#1449).

Validated content: everything documented was exercised live during the #1448/#1449 validation runs (local + b2).

Review Contract

Goal

Add docs/oab-mcp-facade.md: an accurate operator guide for the OAB MCP Facade shipped in #1448 — enablement (broker [mcp] / standalone CLI), the two-tool surface, audit log format, client registration (including the kiro --agent per-agent config gotcha), the loopback trust model, and the pod-vs-fleet positioning relative to octobroker (ADR §4.1).

Non-goals

• No code, config-schema, or behavior changes (docs-only).
• Not a replacement for the ADR — this is the operator-facing quick reference; design rationale stays in docs/adr/oab-mcp-adapter.md.
• No documentation of unreleased features (trash capability, Calendar adapter).

Accepted Residual Risks

• Docs can drift from behavior; mitigated by documenting only what was exercised live during the feat(mcp): OAB MCP Facade MVP — shared openab-mcp crate + broker loopback HTTP server #1448/feat(mcp): native Gmail adapter — §6.5 profile over GA REST as stdio MCP server #1449 validation runs (local + b2 deployment).

Acceptance Criteria

• Every command, config snippet, and log line in the doc matches the shipped feat(mcp): OAB MCP Facade MVP — shared openab-mcp crate + broker loopback HTTP server #1448 implementation (reviewer spot-check against crates/openab-mcp/src/mcp/facade.rs and config.toml.example).
• The trust-model section states loopback-only + no authentication explicitly.
• Cross-links resolve: ADR §4.1/§6, docs/gmail-native.md (feat(mcp): native Gmail adapter — §6.5 profile over GA REST as stdio MCP server #1449).

Follow-ups

• Update the guide when facade endpoint authentication (bearer/socket permissions, an accepted feat(mcp): OAB MCP Facade MVP — shared openab-mcp crate + broker loopback HTTP server #1448 follow-up) ships.
• Add a fleet-deployment example (bot container boot integration) once the b2 pattern is production-ized.