feat(coding-agent): add pluggable operations for remote tool execution

Adds optional operations parameter to create*Tool functions enabling
delegation to remote systems (SSH, containers, etc.):

- ReadOperations: readFile, access, detectImageMimeType
- WriteOperations: writeFile, mkdir
- EditOperations: readFile, writeFile, access
- BashOperations: exec (with streaming, signal, timeout)

Add ssh.ts example demonstrating --ssh flag for remote execution.
Built-in renderers used automatically for overrides without custom renderers.

fixes #564

packages/coding-agent/examples/extensions/README.md

@@ -31,6 +31,7 @@ cp permission-gate.ts ~/.pi/agent/extensions/
 | `hello.ts` | Minimal custom tool example |
 | `question.ts` | Demonstrates `ctx.ui.select()` for asking the user questions |
 | `tool-override.ts` | Override built-in tools (e.g., add logging/access control to `read`) |
+| `ssh.ts` | Delegate all tools to a remote machine via SSH using pluggable operations |
 | `subagent/` | Delegate tasks to specialized subagents with isolated context windows |
 
 ### Commands & UI