feat(scan): add [scan] dirs to set scan roots without implying home

[django23]

Summary

Closes #129.

[scan] extra only ever adds to the home directory, so there was no way to scan a subset of home without also walking all of it. This adds [scan] dirs, an exhaustive list of scan roots where home is scanned only if you list it:

[scan]
dirs = ~/Code
dirs = /Volumes/Work/clients

This is option 1 from the issue's "alternatives considered". The reporter also suggested renaming the existing keys to drop the "extra" concept, but that breaks every existing config for a problem a second key solves cleanly, so [scan] extra keeps its current additive meaning and is untouched.

Behaviour

• Precedence. A positional CLI argument still overrides everything. Below that, dirs wins over extra; setting both warns rather than silently dropping one, since they answer different questions and mixing them is almost certainly a mistake.
• Missing directories. A missing entry warns and is skipped, matching extra (an unmounted volume shouldn't fail a run). But if every listed directory is missing, that exits 1 rather than scanning nothing at all, which would otherwise look like a successful no-op.
• Cache. Everything downstream (the find traversal, mdfind -onlyin, and the path cache's in-scope filter) already keys off ASIMOV_SCAN_DIRS, so narrowing the roots narrows all three. A cache left over from a full-home scan is filtered on read, not re-excluded, and out-of-scope entries stay in the file so widening the scope later brings them back.
• asimov doctor recognises the new key, so a typo is still reported rather than silently ignored.

Global caches under [fixed_dirs] are still excluded regardless of scan roots, consistent with how [skip_paths] already behaves.

Test plan

• make check passes
• Shellcheck clean
• CHANGELOG.md updated (if user-facing)

9 new Bats tests: scanning only the listed directory, multiple entries, home included only when listed explicitly, one-missing-warns, all-missing-errors, dirs beating extra with a warning, the CLI argument overriding dirs, and a cached run narrowing correctly when the roots shrink.

Verified against bash 5.2 locally. Every new empty-array expansion is guarded by a count check, following the existing bash 3.2 convention in this file; the CI matrix covers 3.2 for real.

README documents the new key alongside extra in "Scan more than your home directory".

[huyz]

Proposal with backward-compatibility sounds perfect. Thanks!