v5.7.0

Guided onboarding for the local embedder (RDR-144): the 384-vs-768 choice is
now an informed nx init step, not a silent packaging default.

Added

• nx init guided onboarding. New top-level verb (distinct from
  nx config init). In local mode it presents the embedder choice and records
  it to config.yml local.embed_model: recommended bge-768
  (BAAI/bge-base-en-v1.5, materially better local search, ~140 MB one-time
  download) or bundled minilm-384. Choosing bge-768 adds the [local]
  extra (editable-safe: a uv tool install reinstalls in place; a dev/editable
  tree gets a manual instruction), pre-fetches the model into a stable XDG
  cache (local.fastembed_cache_path, no more re-download to a volatile
  $TMPDIR on reboot), and offers to migrate pre-existing 384-dim collections.
  Cloud mode is a no-op. Flags: --embedder, --yes.
• Safe 384 to 768 collection migration. nx init detects collections
  indexed with the old embedder (which silently return nothing under bge-768)
  and migrates them under a gate-locked protocol: dry-run preview,
  double-confirm, reindex-first, delete-after-verify. The old collection is
  removed only after the new one is verified populated, so a failed reindex
  never loses data. code__ and manual-note collections are reported, never
  auto-deleted; mixed file+note collections require an explicit note-loss
  confirmation and are never migrated under --yes.
• nx doctor embedder advisories. Flags two previously-invisible states:
  running on the default 384 embedder (nudge to nx init), and the degraded
  case where bge-768 was chosen but the [local] extra is missing (so search
  silently runs at 384-dim). The MCP server also surfaces the notice to
  plugin/Desktop/Cowork users via its server instructions.

Fixed

• No orphan state after a collection delete or migration. nx collection delete and the new migration share one cascade (purge_collection_cascade),
  so removing a collection also purges its catalog, taxonomy, chash, and
  pipeline rows; cascade-step failures are surfaced to the user instead of
  swallowed.
• Reindex verification no longer miscounts empty sources. A source file
  that now produces zero chunks can no longer inflate the migration's
  verification and trigger a data-losing delete.

Docs

• Onboarding presents the embedder choice as the primary path across README,
  getting-started, cli-reference, and configuration; supersedes the 5.6.x
  "or install conexus[local]" stopgap framing for the default decision.