feat: DWIM toggle for pi-coding-agent entry point

[conornash]

Add agent-shell-style DWIM behavior to the main pi-coding-agent' command and a new pi-coding-agent-toggle' command for toggling session window visibility.

DWIM behavior for M-x pi-coding-agent (no prefix arg):

• From a pi buffer: toggle visibility (hide windows)
• From a non-pi buffer with existing project session: reuse it
• No session for project: create a new one
• C-u prefix or Lisp string arg: named session (unchanged)

New command `pi-coding-agent-toggle':

• Pi windows visible -> hide them
• Pi windows hidden but session exists -> show them
• No session -> user-error

New functions in pi-coding-agent-ui.el:

• `pi-coding-agent-project-buffers': find all pi chat buffers for the current project directory, ordered by buffer-list recency
• `pi-coding-agent--hide-buffers': hide pi windows using a hybrid strategy: delete-window when the frame has other windows, bury-buffer for sole-window frames. Second pass handles the case where bury-buffer lands on the paired pi buffer.

Bug fix in test-common.el:

• Mock session cleanup called nonexistent functions pi-coding-agent--chat-buffer-name' and pi-coding-agent--input-buffer-name' (swallowed by ignore-errors, causing buffer leaks). Fixed to use `pi-coding-agent--buffer-name' with :chat/:input type argument.

Tests: 5 new tests (502 total, all passing):

• dwim-reuses-existing-session
• new-session-with-prefix-arg
• project-buffers-finds-session
• project-buffers-excludes-other-projects
• toggle-no-session-errors