skills(cmux-rebuild): name the ZELLIJ_SOCKET_DIR base the split-socket recovery needs

[DJRHails]

Follow-up to #162, which merged before its review landed.

Why

The recovery step for a socket-dir split said to read each server's dir off its cmdline and pass it as ZELLIJ_SOCKET_DIR. The cmdline path is <base>/<version-subdir>/<session>, and zellij appends the version subdir to ZELLIJ_SOCKET_DIR itself (zellij-utils/src/consts.rs: ZELLIJ_SOCK_DIR pushes CLIENT_SERVER_CONTRACT_DIR after envs::get_socket_dir()), so passing the cmdline's dirname makes zellij attach look in …/contract_version_1/contract_version_1/ and see nothing. Read literally, the documented recovery does not attach.

What

Spell out that <base> is the dir above the version subdir (/run/user/<uid>/zellij or /tmp/zellij-<uid>), with an example cmdline path so the next reader can see where to cut.

Verified

Read upstream consts.rs, sessions.rs, envs.rs and src/commands.rs (server spawned as <exe> --server <socket_path>). All pre-commit hooks passed (prek), including the prose-lint suite; prose-lint.sh on the file reports only pre-existing hits.

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[DJRHails]

Review Summary

No findings. The doc change is accurate against upstream zellij source (main as of 2026-09-07):

#	Severity	Finding	Resolution
—	—	None	—

Verified:

• zellij-utils/src/consts.rs: ZELLIJ_SOCK_DIR = (ZELLIJ_SOCKET_DIR | ProjectDirs("org","Zellij Contributors","Zellij").runtime_dir() → $XDG_RUNTIME_DIR/zellij | temp_dir()/zellij-<uid>) then push(CLIENT_SERVER_CONTRACT_DIR) = contract_version_1. So <base> is the dir above the version subdir, and passing the cmdline's dirname doubles it exactly as the doc says.
• zellij-client/src/lib.rs spawn_server: <current_exe> --server <ZELLIJ_SOCK_DIR>/<session> — the cmdline does end in <base>/<version-subdir>/<session>. (The PR body attributes this to src/commands.rs; it lives in zellij-client/src/lib.rs, which does not affect the change.)
• zellij-utils/src/sessions.rs: attach/list-sessions resolve through ZELLIJ_SOCK_DIR.join(name) / read_dir(ZELLIJ_SOCK_DIR), so the env ZELLIJ_SOCKET_DIR=<base> zellij attach <session> recovery attaches.
• prose-lint.sh on the file: 3 hits, all pre-existing (em-dash count 44 on both base and head; the two rule-of-three lines 17 and 67 are untouched by this PR).
• No zellij binary in this container, so the recovery command was not exercised live; source reading only.

Fix commit: none pushed
Reviewed head: 15ed7cc

Verdict: approve

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