v1.6.0

TUI and session-management improvements on top of the /btw work.

• /btw live side panel: the main task now resumes the instant a side answer finishes (it keeps rendering on the left while the finished answer stays frozen on the right), the panel persists across turn boundaries and closes only on Esc, and an idle /btw uses the same panel. Esc is state-aware: cancel a streaming answer, dismiss a frozen panel without touching the task, or interrupt the task when no panel is up.
• Single-instance guard: fixes the "cannot load model" crash — the engine's own guard called exit(2) deep in startup; plank now probes the lock file first and exits cleanly with a clear message.
• Session names: sessions get a memorable adjective-celebrity id (e.g. deadly-einstein) minted on first save — 50 adjectives × 150 celebrities (75 scientists / 75 historical-pop-sport), with a short guid on collision. Legacy 40-hex sessions still load and list.
• Resume from the CLI: plank /resume [name] resumes a session at startup (a name, prefix, list number, or bare = most recent); the recovered history is shown.
• End-of-session dump: on exit plank saves the transcript and prints where it landed and how to resume it.
• Prompt separator: a green rule now divides the scrollback from the resting prompt.

Minor bump: the cached system-prompt KV snapshot is invalidated on upgrade.

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