feat: index Codex sessions alongside Claude Code with source tagging

Add `source` column to sessions and messages ('claude' | 'codex').
  Discover and parse Codex JSONL files from ~/.codex/sessions/, mapping
  Codex thread/item structures to the same schema (messages, tool_calls,
  tool_results, subagents). Move DB to ~/.obelisk/ with legacy migration.
  Add rebuild-to-temp-then-swap for safe full rebuilds. On the app side:
  source filter toggle, collapsible untitled session fold, configurable
  codexDir in Settings, and a dev script. Update SKILL.md and query
  helpers to expose source fields and accept source filter opt.

references/retrieval-semantics.md

@@ -33,11 +33,13 @@ unless scoped evidence is insufficient and `query_plan` says why.
 
 Project-like fields are distinct:
 
-- `sessions.project`: stored Claude Code project slug.
+- `sessions.project`: provider-normalized project slug.
 - `memories.project`: stored project slug copied onto registered memory records.
 - `sessions.project_path`: absolute session path derived from message `cwd` when available; slug decoding is only a fallback.
 - `messages.cwd`: working directory at message time.
+- `sessions.source` / `messages.source`: transcript provider, currently `claude` or `codex`.
 - helper `project`: SQL `LIKE` over `sessions.project`, not exact membership.
+- helper `source`: optional provider filter. Omit it unless provenance matters.
 
 For exact project membership, prefer helper filters or a scoped first pass when
 they are expressive enough; use `sql()` with `s.project = ?` or