feat: add k keybinding to kill selected session

[robinojw]

Summary

• Add k keybinding that kills and removes the selected agent session from the canvas
• Stops PTY process, unpins from session panel, deletes from store, and clamps selection
• Works in both grid and tree canvas modes
• Adds ClampSelected() to CanvasModel for safe index adjustment after deletion
• Adds kill keybinding entry to the help screen

Test plan

• ClampSelected clamps index after deletion and handles empty store
• k removes thread from store, stops PTY, unpins session, clamps selection
• k with no threads is a no-op
• k returns focus to canvas when last pinned session is killed
• k works in tree mode via selectedThreadID() helper
• Help screen shows kill keybinding
• Full suite passes with race detector (go test ./... -v -race)
• Lint clean (golangci-lint run — 0 issues)