fix: stop a background refresh undoing delete/prune/rename

[brtkwr]

Fixes from an adversarial review of the auto-refresh change:

• Deletes, prunes and renames stay put. A background refresh that started before you deleted, pruned or renamed a conversation used to land afterwards and put the old state back for up to a minute. A deleted row could reappear and fail to resume. That refresh is now thrown away, and the next one picks up the real state.
• A failed refresh keeps the list. Before, if ~/.claude/projects went missing or couldn't be read, the refresh emptied the list. It now counts as a failure and the current list stays.
• Cheaper refreshes. Conversations whose file hasn't changed keep their search text instead of having it rebuilt. The reviewer measured that rebuild at about 64MB and 173ms every minute, and it no longer runs for unchanged files.
• Live sessions: prune is refused while the session is open in claude, because anything claude writes during the rewrite would be lost. Delete still works but the confirmation says the session is open.
• Deleted files are also removed from the parse cache.

Not changed:

• A hung scan stopping refreshes: a separate timer can't recover from that either, because the stuck scan never finishes.
• Using the session file's recorded start time to rule out a reused pid: left for later.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JUmkVYwbkobda1AnHuwFDE