kdbx: complete operations docs + add /kdbx:install-launcher command

[nabsha]

Gaps found reviewing the operations table:

• install-launcher is missing from the README + SKILL.md operations tables (it's the 13th op).
• --allow-missing (on run/export) is undocumented.
• delete --purge / rekey now prompt interactively (y/N) — the tables don't mention it.
• Add a /kdbx:install-launcher slash command — there's no command for it today (the agent can run it — it's allowlisted by the role-guard — but it isn't discoverable in the / menu like /kdbx:init).

Small doc + one command file; no core changes.