Support pointing cq at a single arbitrary session jsonl file

The problem

cq only knows how to scan sources: directory roots shaped like <projects_dir>/<project>/**/*.jsonl (main, plus discovered cenv envs). There's no way to hand cq one specific transcript file that lives outside those roots, like one someone sent you, or one pulled from a different machine/backup.

Current workaround

collect_jsonl in src/indexer.rs recurses arbitrarily deep and picks up any *.jsonl under a source root regardless of naming, so you can fake it:

mkdir -p /tmp/cq-adhoc/adhoc
ln -s /path/to/session.jsonl /tmp/cq-adhoc/adhoc/session.jsonl
CQ_PROJECTS_DIR=/tmp/cq-adhoc cq sessions --all

Works, but it's not something you'd guess from the CLI, and it's easy to forget the file needs a directory level above it (a bare CQ_PROJECTS_DIR=/tmp/cq-adhoc pointed straight at the file won't scan, since scan_all expects one level of project dirs before it recurses).

Proposal

A direct way to index one file, something like:

cq sessions --file /path/to/session.jsonl

or a lighter-weight one-off source registration (cq sources add <path> / --source-path) that doesn't require the symlink dance. Either would remove the need to know the internal directory-shape requirement to do something that's really just "look at this one file."

Start in src/indexer.rs, reading collect_jsonl and scan_all to understand the current source-root assumptions, then trace the cq sessions command's argument handling. Add a direct way to index one arbitrary session.jsonl file without the project-directory workaround, and verify that the selected transcript is scanned successfully.