feat: add single-pane mode (--no-exec-pane) for chat-only usage

[alvinreal]

Summary

Adds --no-exec-pane CLI flag and tmux.no_exec_pane config option to run TmuxAI without creating a second exec pane. Addresses #143.

Why

Users want to use TmuxAI in custom layouts (popup windows, bottom bars, split arrangements) where a second exec pane is undesirable or impossible.

What changes

• CLI: new --no-exec-pane flag
• Config: new tmux.no_exec_pane: true option
• NewManager: skips InitExecPane() when flag is set
• process_message.go: guards ExecCommand, SendKeys, PasteMultilineContent — logs a message and skips when no exec pane
• prompts.go: new singlePanePrompt() — tells the AI it can only chat, not execute
• chat_command.go: /prepare and /reset handle missing exec pane gracefully
• pane_details.go: exec pane references guarded
• config_helpers.go: GetNoExecPane() helper + allowed config key

Behavior in single-pane mode

• AI can chat, answer questions, provide instructions
• AI cannot execute commands, send keys, or paste content
• Users get a clear message when they try operations that require an exec pane

Testing

• golangci-lint run → 0 issues
• go test ./... → passes

Closes #143