feat(deepagents): implement default middleware systemprompt override

[Oscar-Umana]

Summary

Adds a middlewareSystemPrompts option to createDeepAgent so callers can override system prompts injected by the built-in middleware stack.

Supported overrides:

• todoList
• filesystem
• subagent
• asyncSubagent

The new DeepAgentMiddlewareSystemPrompts interface is exported from the package entrypoint for consumers who want to type config objects directly.

Changes

• Adds middlewareSystemPrompts?: DeepAgentMiddlewareSystemPrompts to CreateDeepAgentParams.
• Wires prompt overrides into the built-in root middleware stack.
• Applies the same todo/filesystem prompt overrides to normalized subagent middleware.
• Supports disabling the subagent task prompt with subagent: null, matching existing createSubAgentMiddleware behavior.
• Adds runtime coverage that verifies custom middleware prompts replace the defaults.
• Adds a type test confirming the new param is accepted by createDeepAgent.

Validation

• pnpm --dir libs/deepagents exec vitest run src/agent.test.ts --config vitest.config.ts --typecheck.enabled=false
• Editor diagnostics pass for the edited files.

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commit: 45ed871

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Latest commit: 45ed871

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[Oscar-Umana]

Hi Christian Bromann (@christian-bromann) what would be the process to get some reviews on this PR?