feat: add alive:end-session skill for explicit session closing

[0xmoostorm]

Summary

• Adds /alive:end-session — an explicit command to cleanly close a session
• Routes unsaved stash, writes closing log entry, sets ended: timestamp on squirrel YAML
• Updates save skill to delegate session exit to end-session instead of inlining the protocol
• Registers as the 16th skill in CLAUDE.md

Problem

Sessions that end by closing the terminal, disconnecting, or just walking away never get their squirrel entry closed. The ended: field stays null forever. Over time these ghost squirrels accumulate — polluting session history, confusing system-cleanup, and causing fallback patterns in hooks (grep -rl 'ended: null') to pick up wrong sessions.

A SessionEnd hook was considered but rejected: it would break session resume, since the resume hook looks for ended: null to find active sessions. An explicit skill is the right design — the human decides when they're truly done.

Changes

File	Change
plugins/alive/skills/end-session/SKILL.md	New skill — full flow with stash check, closing log, squirrel update
plugins/alive/CLAUDE.md	Register as 16th skill
plugins/alive/skills/save/SKILL.md	"On Actual Session Exit" now delegates to end-session

Test plan

• Start a session, stash some items, run /alive:end-session — verify stash routing prompt appears
• Run /alive:end-session with no stash — verify it skips straight to closing
• Check squirrel YAML after close — ended: should have ISO timestamp
• Run /alive:end-session on an already-closed session — should print "session already closed"
• Resume a session that was NOT end-sessioned — verify resume still works (regression check)

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