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/index.md

@@ -12,6 +12,8 @@ start with install, then follow the section that matches your task.
 | Goal | Read |
 |---|---|
 | Install OmniGraph | [install.md](install.md) |
+| Run the core loop end to end | [quickstart.md](quickstart.md) |
+| Understand the model | [concepts/index.md](concepts/index.md) |
 | Run the CLI | [cli/index.md](cli/index.md) |
 | Look up every CLI flag and config field | [cli/reference.md](cli/reference.md) |
 
@@ -21,8 +23,9 @@ start with install, then follow the section that matches your task.
 |---|---|
 | Write schemas (the `.pg` language) | [schema/index.md](schema/index.md) |
 | Read schema-lint diagnostic codes | [schema/lint.md](schema/lint.md) |
-| Write queries and mutations (the `.gq` language) | [queries/index.md](queries/index.md) |
-| Use vector / full-text / hybrid search | [search/indexes.md](search/indexes.md) |
+| Write queries (the `.gq` language) | [queries/index.md](queries/index.md) |
+| Write data — inserts, updates, deletes | [mutations/index.md](mutations/index.md) |
+| Use vector / full-text / hybrid search | [search/index.md](search/index.md) |
 | Generate embeddings | [search/embeddings.md](search/embeddings.md) |
 | Build and use indexes | [search/indexes.md](search/indexes.md) |
 
@@ -30,7 +33,9 @@ start with install, then follow the section that matches your task.
 
 | Goal | Read |
 |---|---|
-| Work with branches, commits, and snapshots | [branching/index.md](branching/index.md) |
+| Work with branches and commits | [branching/index.md](branching/index.md) |
+| Read past versions (time travel) | [branching/time-travel.md](branching/time-travel.md) |
+| Merge branches and resolve conflicts | [branching/merge.md](branching/merge.md) |
 | Coordinate multi-query workflows | [branching/transactions.md](branching/transactions.md) |
 | Read diffs and change feeds | [branching/changes.md](branching/changes.md) |
 
@@ -56,6 +61,7 @@ start with install, then follow the section that matches your task.
 
 | Goal | Read |
 |---|---|
+| Understand the model and L1/L2 framing | [concepts/index.md](concepts/index.md) |
 | Understand graph layout and URI support | [concepts/storage.md](concepts/storage.md) |
 | Look up constants and tunables | [reference/constants.md](reference/constants.md) |