docs(user): restructure user docs into topic sections (Phase 1) (#223)

Move the 23 flat docs/user/*.md files into topic subdirectories so the
user guide is organized by area (schema, queries, search, branching, cli,
operations, clusters, concepts, reference) instead of a flat list. This is
a pure structural move — whole files relocated, every cross-doc link
recomputed, no prose rewrites or content splits (those follow in Phase 2).

- 19 `git mv`s (install.md, deployment.md stay top-level); history preserved
  (renames detected at 92–100% similarity).
- All intra-doc links, AGENTS.md's topic table (52 pointers), and the
  docs/dev + docs/releases back-links recomputed via relpath from each
  file's new location.
- docs/user/index.md rewritten as a sectioned nav hub.
- Fixed 5 doc-path references in Rust (comments + two user-facing server
  settings error strings) to point at the new locations.

Verified: zero broken .md links across tracked docs; check-agents-md.sh
green (with the untracked scratch docs set aside); touched crates build.

Note: the public site (omnigraph-web) imports docs/ via a flat-only script;
its import-docs.mjs needs a subdir-aware update before the next re-sync.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/dev/writes.md

@@ -305,7 +305,7 @@ success and one failure. The losing writer's error is
 `ManifestConflictDetails::ExpectedVersionMismatch { table_key, expected,
 actual }`. The HTTP server maps this to **409 Conflict** with body
 `{"error": "...", "code": "conflict", "manifest_conflict": { "table_key":
-"...", "expected": N, "actual": M }}` — see [docs/user/server.md](../user/server.md).
+"...", "expected": N, "actual": M }}` — see [docs/user/server.md](../user/operations/server.md).
 
 ## Audit