fix: follow up on #125 to make pi-coding-agent behavior predictable

[dnouri]

Follow-up to #125 by @conornash.

#125 introduced useful DWIM/toggle ideas, but also made M-x pi-coding-agent surprising in a few real workflows. This PR keeps the split command model and smooths out those rough edges.

What changed

• M-x pi-coding-agent is now consistently a show/restore/focus command for the current session.

  • If chat+input are already visible, layout is left alone and focus goes to input.
  • If one pane is missing, the split is restored.

• M-x pi-coding-agent-toggle is now the explicit hide/show command.

  • Visibility decisions are made per current frame (not “any frame”).
  • If only one pane (e.g. input) is visible, toggle still hides the session cleanly.

• Session lookup is now based on exact normalized project directory, which avoids selecting the wrong session (e.g. parent vs subdir sessions).

• Window restore behavior was tightened to avoid duplicate input panes and to pick/focus the correct session input window when multiple sessions are visible.

• Docs were updated to match actual command semantics and clarify pi-coding-agent vs optional pi alias usage.