Add support for appending or overriding the system prompt

[WhiskeyJack96]

resolves #90

Tested both the append and override flows by sending:

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: []
		});

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!

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[WhiskeyJack96]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[WhiskeyJack96]

Typescript isnt my strong suit, so if theres a more idiomatic way to handle the union disambiguation I'm happy to give it a go!

[benbrandt]

@WhiskeyJack96 this looks great! I made the checks slightly stricter to be safe, but otherwise it makes sense to me!