skills(cmux-rebuild): document the zellij socket-dir split that under-counts live sessions

[DJRHails]

Companion to #161. The rebuild survey reported 2 live taffy sessions while 12 zellij servers were running: logind at SessionsMax left new logins without XDG_RUNTIME_DIR, so zellij list-sessions read /tmp/zellij-1000 and every server under /run/user/1000/zellij looked EXITED. This adds the check (compare the survey with the server process count over ssh) and the recovery (attach with env ZELLIJ_SOCKET_DIR=<dir>; symlinks between the dirs do not work) to the skill, where the next rebuild reads it.

via claude @ trifle

[DJRHails]

Automated review — one finding posted inline. The PR had already merged, so the fix is a follow-up PR off main rather than a push to this branch.

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[DJRHails]

Review Summary

#	Severity	Finding	Resolution
F1	P2	Recovery step passes the server cmdline's dirname as ZELLIJ_SOCKET_DIR, but zellij appends the version subdir itself, so the documented zellij attach looks in …/contract_version_1/contract_version_1/ and finds nothing	Fixed in PR #163 (15ed7cc); this PR had already merged
—	P4	Live-count anecdote says 2 of 12 servers here and 2 of 11 in #161's description; not verifiable from the repo	Noted only

Everything else in the paragraph checks out against upstream zellij (zellij-utils/src/consts.rs, sessions.rs, envs.rs, src/commands.rs): socket dir is $ZELLIJ_SOCKET_DIR → $XDG_RUNTIME_DIR/zellij → temp_dir()/zellij-<uid>, servers run as <exe> --server <socket_path>, and list-sessions skips symlinks because it tests the DirEntry's own file type.

Verified: read the merged diff and the surrounding SKILL.md section plus the rebuild scripts' list-sessions calls; read the four upstream zellij sources above via the GitHub API. For the fix: all prek pre-commit hooks passed (gitleaks, trufflehog, glassine checks, the test suites including prose-lint's); prose-lint.sh on the file reports only pre-existing hits. No CI on this repo runs the hooks, so the hook run is the verification.
Fix commit: 15ed7cc on docs/cmux-rebuild-socket-dir-base (PR #163, left open for the author to merge)
Reviewed head: 04f97b4 (merged as 29a575a)

Verdict: approve

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