Add cancel/abort for in-flight LLM requests and tool executions

[bug-ops]

Problem

Currently the only way to stop an ongoing LLM request or tool execution is Ctrl+C (full shutdown). There is no way to cancel a single operation while keeping the agent running.

Use case: user sends a wrong prompt and wants to abort the LLM response or tool chain mid-flight without restarting.

Current state

• watch::channel<bool> shutdown signal — binary, triggers full agent exit
• Streaming path uses tokio::select! with shutdown — works but kills everything
• Native tool loop checks *self.shutdown.borrow() per iteration — same binary flag
• Parallel tool execution (join_all / buffered) has no per-operation cancellation
• TUI has no cancel keybinding; CLI has no interrupt signal besides Ctrl+C

Proposed approach

1. Add CancellationToken (from tokio-util) per operation

Replace the binary shutdown watch with a two-level scheme:

• Operation-level CancellationToken — cancels current LLM call / tool chain, agent stays alive
• Session-level shutdown — existing behavior (Ctrl+C), also triggers operation cancel via drop_guard

2. Wire cancellation into agent loop

• process_response_native_tools(): wrap join_all/buffered tool futures with token.cancelled()
• process_response_streaming(): replace shutdown branch with operation token
• call_llm_with_timeout(): add token to tokio::select!
• Shell executor: kill child process on cancel (child.kill())

3. Channel trait: add cancel signal

// Channel trait addition
fn cancel_rx(&mut self) -> Option<&mut tokio::sync::watch::Receiver<bool>>;

Or pass CancellationToken into agent context so it's channel-agnostic.

4. TUI: Esc in Normal mode during active operation → cancel

• When agent is processing (spinner visible), Esc sends cancel signal
• Show [Cancelled] status in chat history
• Agent returns to idle, ready for next input

5. CLI: Ctrl+C once → cancel operation, twice → exit

• First Ctrl+C cancels current operation
• Second Ctrl+C (within 2s) triggers full shutdown
• Standard double-Ctrl+C UX pattern

Scope

• zeph-core: CancellationToken plumbing in agent loop and streaming
• zeph-tools: shell child process kill on cancel
• zeph-tui: Esc keybinding + cancel indicator
• zeph-channels: CLI double-Ctrl+C handler

Acceptance criteria

• Esc in TUI cancels current LLM/tool operation without exiting
• Double Ctrl+C in CLI: first cancels, second exits
• In-flight HTTP requests (LLM) are dropped on cancel
• Running shell commands are killed on cancel
• Chat shows [Cancelled] for aborted operations
• Agent resumes normal input loop after cancel