feat: copyable shell commands and persistent footer toolbar

[michaeljabbour]

What this PR does

Two quality-of-life improvements to the interactive CLI that make daily use noticeably better.

✂ Copyable Shell Commands

Shell commands in agent responses are now rendered in a dedicated "Copy & Run" frame. Triple-click to select the entire command — no more fighting with line continuations or partial selections.

How it works:

• Markdown AST parsing (via markdown-it-py) detects shell code fences by language tag
• Backslash line continuations are collapsed into single logical lines
• Commands render with soft_wrap=True so terminal wrapping never introduces \n into the copied text
• A developer system message instructs the model to emit single-line commands (new sessions only)

Non-shell code blocks render exactly as before — the fast path is preserved.

📊 Persistent Footer Toolbar

A centered status bar now lives at the bottom of the terminal during interactive chat:

              [mode] a1b2c3d4 | claude-sonnet-4-20250514 | ~/dev/my-project

• Active mode (when set), truncated session ID, model name, and working directory
• HTML-escaped dynamic values for safety
• Centered layout that adapts to terminal width

Files changed

File	What
amplifier_app_cli/main.py	Footer toolbar, developer message injection (gated on new sessions)
amplifier_app_cli/ui/message_renderer.py	Copyable shell command rendering with safe fence parsing
uv.lock	Add pygments dependency