locking: fix lock-listing race and with-lock epilog

[ThomasWaldmann]

Two independent storelocking fixes, split out of #10077 (both found during its review, both pre-existing on master and valid regardless of the clock-skew work):

locking: ignore locks vanishing between listing and loading them — _get_locks() guarded store.list() against ObjectNotFound, but not the per-lock store.load(). Two clients racing on stale-lock cleanup could therefore crash with a raw borgstore traceback: one deletes a stale lock while the other is between listing the locks directory and loading that object. Such locks are now skipped — semantically identical to a lock already gone at listing time. With a test simulating the race.

with-lock: fix epilog about leftover lock auto-removal — the help epilog claimed "Borg is cautious and does not automatically remove stale locks made by a different host", which is not true for borg2 storelocking: foreign stale locks are auto-removed after the stale timeout. The epilog now says the leftover lock in a repository copy is removed automatically after a safety period of about half an hour, and that borg break-lock is the way to use the copy right away. Regenerated docs/usage/with-lock.rst.inc and docs/man/borg-with-lock.1 accordingly.

#10077 will be rebased to drop its copies of these two commits once this is merged.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.01%. Comparing base (8640a75) to head (e15404a).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10096   +/-   ##
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  Coverage   87.00%   87.01%           
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  Files          99       99           
  Lines       17612    17615    +3     
  Branches     2664     2664           
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+ Hits        15323    15327    +4     
  Misses       1593     1593           
+ Partials      696      695    -1     

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