Rework AI terminal commands with VS Code-style shell integration

[hendrikmennen]

Follow-up to #322. Full rework of how AI terminal commands are tracked, replacing the out-of-band completion channel with VS Code-style shell integration.

Terminal command tracking (OSC 633 shell integration)

The previous approach typed a visible marker command into the shell, suppressed its echo with cursor-erase hacks, and signalled completion through extra pipes/handles — which sometimes leaked the marker into the user-facing terminal and behaved differently across Unix/Windows.

Now, shell startup files install prompt hooks that emit invisible, in-band OSC 633 sequences (C = command started, D;<exit> = command finished with exit code), exactly like VS Code's shell integration:

• ShellIntegration.cs — generates integration scripts for bash (--init-file), zsh (ZDOTDIR) and PowerShell, sourcing the user's own startup files first. Scripts are written atomically to a per-user directory (~/.oneware/shell-integration, 0700).
• ShellIntegrationParser.cs — chunk-safe streaming parser that strips OSC 633 sequences from pty output and raises typed events; all other escape sequences pass through untouched.
• TerminalManagerViewModel — captures output strictly between the C/D markers (no prompt drawing, no command echo) with a short grace period so multi-line command blocks complete as one execution.
• Nothing is ever typed into the shell and no extra fds/pipes are inherited; the same mechanism works on Unix and Windows (ConPTY).
• User input remains possible while a command runs; a best-effort UserInputEchoFilter keeps echoed keystrokes out of the captured/streamed tool output (visible terminal unaffected).

Robustness fixes

• Shell exit handling: exit/crash of the shell no longer hangs pending executions — pty EOF raises Closed, the real process exit code is captured (waitpid on Unix, Process.ExitCode on Windows) and reported, and the terminal tab closes automatically.
• forkpty deadlock: pre-fork P/Invoke warm-up fixes a latent hang where the first spawned terminal could deadlock in the child before execve.
• Timeout/cancel recovery: gentle Ctrl+C first; if the shell stays stuck, the process tree is killed and the automation terminal discarded instead of being reused.

Chat UX

• Stop button on every running tool call (right of the spinner) — cancels just that invocation via a per-invocation CancellationTokenSource (IAiFunctionProvider.CancelFunction); the model receives a graceful "stopped by user" result instead of the turn failing.
• Chat abort now also cancels running tool invocations.
• Queued messages: item template with remove button, plus IChatService queue management (RemoveMostRecentQueuedMessageAsync/ClearQueuedMessagesAsync) wired to the Copilot SDK.
• Copilot CLI bumped to 1.0.74.

Tests

New/rewritten coverage: parser strip/event/chunk-split tests, spawn-config tests per shell, echo-filter tests, and live-bash integration tests (command lifecycle, failing exit codes, exit 3 closing the session with the real exit code, no marker/control-byte leakage). 22 tests passing.

Net result: −460 lines while covering more edge cases.

Submodule cleanup: hendrikmennen/VtNetCore.Avalonia#2