Add support for appending or overriding the system prompt - #91

…tprotocol#91)

resolves agentclientprotocol#90 

Tested both the `append` and `override` flows by sending:
```javascript
const resp = await this.connection.newSession({
			cwd: basePath,
			_meta: {
				systemPrompt: "you are an obsidian assistant agent, you should be helpful to the user when working with their notes"
			},
			mcpServers: []
		});
```

```javascript
const resp = await this.connection.newSession({
			cwd: basePath,
			_meta: {
				systemPrompt: {
					append: "always speak in spanish"
				}
			},
			mcpServers: []
		});
```

and confirmed that in both cases those prompts are what gets sent to the
agent!

---------

Co-authored-by: Ben Brandt <[email protected]>

…ow (agentclientprotocol#91)

the agent to use the sandbox

…P _meta

Adds global flags --system-prompt <text> and --append-system-prompt <text>
that forward through ACP session/new _meta.systemPrompt, using the shape
landed in agentclientprotocol/claude-agent-acp#91:

  _meta.systemPrompt = "..."                 # replace
  _meta.systemPrompt = { append: "..." }     # append

The value is persisted in session_options.system_prompt so
acpx claude sessions ensure / reuse flows keep the override, and rides
alongside the existing claudeCode options in _meta. Codex and other
agents ignore the field.

Mutual exclusion between the two flags is enforced at resolve time.

…P _meta

Adds global flags --system-prompt <text> and --append-system-prompt <text>
that forward through ACP session/new _meta.systemPrompt, using the shape
landed in agentclientprotocol/claude-agent-acp#91:

  _meta.systemPrompt = "..."                 # replace
  _meta.systemPrompt = { append: "..." }     # append

The value is persisted in session_options.system_prompt so
acpx claude sessions ensure / reuse flows keep the override, and rides
alongside the existing claudeCode options in _meta. Codex and other
agents ignore the field.

Mutual exclusion between the two flags is enforced at resolve time.

…P _meta

Adds global flags --system-prompt <text> and --append-system-prompt <text>
that forward through ACP session/new _meta.systemPrompt, using the shape
landed in agentclientprotocol/claude-agent-acp#91:

  _meta.systemPrompt = "..."                 # replace
  _meta.systemPrompt = { append: "..." }     # append

The value is persisted in session_options.system_prompt so
acpx claude sessions ensure / reuse flows keep the override, and rides
alongside the existing claudeCode options in _meta. Codex and other
agents ignore the field.

Mutual exclusion between the two flags is enforced at resolve time.