feat(commands): Filter command completion by ArgLead and add delete-session picker

[mhiro2]

Summary

• Add prefix filtering to user command completion so typing a partial argument narrows the candidate list — Lua-function completion is customlist-style, which Neovim does not filter against ArgLead on its own.
• Make PeekstackDeleteSession accept an optional name and fall back to a vim.ui.select picker of saved sessions when invoked without one, matching PeekstackListSessions.

Changes

• 859fc5c : feat(commands): filter command completion by ArgLead and add delete picker
  • Add a filter_by_prefix helper and apply it to PeekstackRestoreSession, PeekstackDeleteSession, and PeekstackQuickPeek completions; QuickPeek candidates keep the registry's sorted order since filtering only narrows.
  • Change PeekstackDeleteSession to nargs="?" so a nameless invocation lists saved sessions for selection before confirming deletion, sharing a confirm_delete_session helper with the named path.
  • Update README and the help doc for the optional [name] argument and add command spec coverage for prefix filtering and the delete picker flow.