fix: linter was dead in a personal-store-only project

[aoc81]

The bug

verify_refs.main() bailed whenever the team store directory was missing, even
with personalStoreDir set and populated:

$ python plugin/scripts/verify_refs.py --stats
No learnings store at .../learnings - run /lore:init first.   (exit 0)

Same for the default mode, --strict, --report and --dupes.

Personal-store-only is a supported setup — it is how you keep notes you do not
want published — and in it recall and capture keep working. So the store looks
healthy while /lore:lint, /lore:stats and /lore:sweep are all silently
gone. The printed advice was wrong too: the project is initialized, just
personal-only.

The early return predates the personal store and was never revisited when it
landed.

The fix

Give up only when both stores are absent. The traversal was already correct —
creating an empty learnings/ directory made every mode work, and --stats then
reported personal store: 2 entries in .lore/personal plus per-entry recall
activity — so this is the guard clause only.

Scope decision

Deliberately unchanged: the personal store still feeds --stats and nothing
else, for the reasons documented in _personal_entries (not committed, so
nothing to index and no push boundary to secret-scan). scan_secrets.py is
untouched — skipping the personal store there is correct by design.

Two consequences are handled explicitly rather than left implicit:

• --index becomes a no-op with a message when there is no team store, instead
  of writing a README into a directory that does not exist.
• The check mode no longer prints "all entries have valid frontmatter file
  refs" after examining zero entries. A clean bill of health for a store nobody
  checked is the same failure this PR fixes, one level down.

Tests

Six cases in tests/test_verify_refs.py drive main() through a personal-only
project via CLAUDE_PROJECT_DIR. Five fail without the guard change; the sixth
pins the message for a populated team store so existing output cannot drift.
Full suite: 168 tests, OK.

Verified exit codes

scenario	exit
--strict, broken ref in team store	1
same without --strict	0
--strict, personal-only	0