Add -D/--delete flag to remove paths from the store

[wyne]

Summary

• Adds -D/--delete flag (accepts multiple values) to remove one or more entries from ~/.fasder
• Implements DeleteFromStore(paths []string) in filestore.go using the same atomic write pattern as the rest of the store
• No-op when a path isn't found (safe to call with stale/typo paths)

Closes #12

Test plan

• fasder -D /some/path removes the entry from the store
• fasder -D /path1 -D /path2 removes multiple entries at once
• Deleting a path not in the store leaves the store unchanged
• go test -run TestDelete ./... passes

https://claude.ai/code/session_015fMoaAdGN1KxxrVHskgicZ

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