docs(user): split language/branching pages + add front-door pages (Phase 2) (#225)

Content build-out on top of the Phase 1 topic move. No behavior changes.

Splits (existing content relocated, cross-linked):
- queries/index.md → mutations/index.md (insert/update/delete + the
  inserts-vs-deletes rule) and search/index.md (the multi-modal search
  functions + a hybrid-ranking overview tying nearest/bm25/rrf together).
  queries/index.md now covers the read shape and points at both.
- branching/index.md → branching/time-travel.md (snapshots/time travel) and
  branching/merge.md (three-way merge + the 7 conflict kinds, verified against
  error.rs MergeConflictKind).

New pages (written from the code, user-facing):
- quickstart.md — init → load → query → branch, with verified CLI flags.
- concepts/index.md — what OmniGraph is + the L1/L2 (Lance/OmniGraph) framing.

Expanded operations/audit.md from a 7-line struct dump into a real
actor-tracking page (server token-resolved vs CLI --as chain; reading the
trail; the omnigraph:recovery reserved actor).

Index wiring: docs/user/index.md and AGENTS.md's topic table link every new
page; also normalized AGENTS.md's docs/user link display text to match the
Phase 1 retargeted paths.

Verified: zero broken .md links; check-agents-md.sh green (57 links, 54 docs).

Deferred to Phase 3: de-dev polish (grammar paths, IR internals still in
queries/branching), guides/, and a possible reference/config.md split (the
config schema is already coherent in cli/reference.md).

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

docs/user/quickstart.md

@@ -0,0 +1,81 @@
+# Quickstart
+
+This walks the core loop end to end: define a schema, initialize a graph, load
+data, query it, and use a branch. It uses a local file-backed graph; swap the
+path for an `s3://…` URI to run the same flow against object storage.
+
+[Install](install.md) the `omnigraph` CLI first.
+
+## 1. Write a schema
+
+A schema (`.pg`) declares your node and edge types. Save this as `schema.pg`:
+
+```
+node Person {
+  name: String,
+  title: String?,
+}
+```
+
+See the [schema language](schema/index.md) for types, constraints, and edges.
+
+## 2. Initialize the graph
+
+```bash
+omnigraph init --schema schema.pg graph.omni
+```
+
+`init` creates an empty graph at the given URI with your schema applied.
+
+## 3. Load data
+
+`load` is the single bulk-write command. `--mode` is required
+(`overwrite | append | merge`):
+
+```bash
+omnigraph load --data people.jsonl --mode overwrite graph.omni
+```
+
+`people.jsonl` is newline-delimited JSON, one record per line. For finer-grained
+or inline writes, see [mutations](mutations/index.md).
+
+## 4. Query
+
+Write a query (`.gq`) — save as `queries.gq`:
+
+```gq
+query find_people($title: String) {
+  match { $p: Person { title: $title } }
+  return { $p.name }
+}
+```
+
+Run it:
+
+```bash
+omnigraph read --query queries.gq --name find_people \
+  --params '{"title":"Engineer"}' --format table graph.omni
+```
+
+The [query language](queries/index.md) covers `match`/`return`/`order`, and
+[search](search/index.md) covers vector and full-text search.
+
+## 5. Work on a branch
+
+Branches isolate changes until you merge them — Git-style, across the whole graph:
+
+```bash
+omnigraph branch create review/new-hires graph.omni
+omnigraph load --data new-hires.jsonl --mode append --branch review/new-hires graph.omni
+# inspect the branch, then integrate it
+omnigraph branch merge review/new-hires --into main graph.omni
+```
+
+See [branches & commits](branching/index.md) and [merging](branching/merge.md).
+
+## Next steps
+
+- [CLI reference](cli/reference.md) — every command and flag.
+- [Schema language](schema/index.md) and [query language](queries/index.md).
+- [Operating a cluster](clusters/index.md) and [running the server](operations/server.md)
+  for multi-graph, multi-user deployments.