Documentation: v5.1 release coverage

Umbrella / parent issue tracking documentation coverage for the **v5.1 (Lincoln)** release. Part of the docs project-board organization — repo-scoped workstreams live here (in `documentation`), not in a separate tracker.

**Purpose:** ensure every v5.1 feature has corresponding reference/learn documentation, and give a single place to see outstanding v5.1 docs work. Release-bound docs issues are attached here as sub-issues; anything that must ship with the release also gets the `v5.1` milestone.

**v5.1 surface** (per [release notes](release-notes/v5-lincoln/5.1.md)):
- AI Models integration (`@embed` directive, Agent Loop)
- MCP server (operations/application profiles, CLI)
- Application routing & middleware (`urlPath`, `before`/`after` ordering, Node.js middleware adapter)
- Deployment tracking (`list_deployments`, `get_deployment`, …)
- HNSW int8 quantization
- Replication improvements
- HTTP caching, `LOCAL_ONLY` writes
- Configuration (`HARPER_CONFIG`, RocksDB memory, `migrateOnStart`)
- v4→v5 upgrade fixes

**Status:** v5.1 features appear largely documented already; this Initiative captures any remaining gaps as they're identified.

Start with release-notes/v5-lincoln/5.1.md and review the v5.1 sub-issues in the documentation project board. Check each listed feature for corresponding reference and learn documentation, then confirm remaining gaps are covered and release-bound work is marked for v5.1.

Written by the indexing model from the issue text.