Need a way to programmatically start a session/subagent with specific context window and reasoning effort

[kuwii]

Describe the feature or problem you'd like to solve

Currently model's context window size and reasoning effort can only be manually specified through the interactive /model flow. There seems to be no way to specify it for subagent and SDK.

Proposed solution

• Agent markdown file's frontmatter adds some properties to specify the reasoning effort and context window level.
• Copilot SDK exposes a supported way to specify reasoning effort and context window tier when creating a session.

Example prompts or workflows

• Although some models have larger context window (like 1M), but by default they use a smaller one (like 200K), which limits their performance.
• For automation system, it needs a more powerful model with more reasoning effort and larger context window to do complex tasks for large project.
• Multi-agent workflow requires delegating tasks to subagent, and may need different subagent to have different settings.

Additional context

No response

[tyrielv]

"effortLevel" configuration exists, but +1 on adding similar configuration for context tier. My favorite model recently split into tiers within the same model instead of having separate models with slightly different names, and not being able to set the longer context by default is bothersome unless something like #3530 is implemented.