Create() now registers a Session with Core=nil instead of just reserving
a name. All code paths that accessed sess.Core are nil-guarded:
- root.go: buildIndex, InterruptAll, rootOpen (proc read), rootDel (proc dismiss)
- persist.go: Kill, Rename, Shutdown, waitIdle, persistSession
- files.go: handleCtl save command
- create.go: Create() creates and registers the empty session
Also fixed pathType in server.go which panicked on empty sessions
(calling sess.Core.Agent().Name() when Core is nil). The panic was
caught by the recovery handler and returned as 'internal error'.
AGENTS.md: document cmd/ollie-remote test/build quirk
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