Introduce proposed `agentsWindow` configuration extension point (#314575)

* Introduce proposed agentsWindow configuration extension point

Add a new `agentsWindow` property to the configuration contribution point
that allows extensions to declare per-setting default value overrides and
read-only behavior for the Agents window.

Shape: `agentsWindow: { default?: unknown; readOnly?: boolean }`

- Gated behind `agentsWindowConfiguration` proposed API
- SessionsDefaultConfiguration uses agentsWindow.default as the default value
- ReadOnly settings are excluded from user configuration parsing and
  cannot be written via updateValue
- Settings editor shows lock indicator for readOnly settings
- `@override:agentsWindow` filter in settings search (agents window only)
- Adopted all existing hardcoded session defaults to use the new schema

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* revert

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>