feat(agents): support thinkingDefault: "adaptive" for Anthropic models (#31227)

* feat(agents): support `thinkingDefault: "adaptive"` for Anthropic models

Anthropic's Opus 4.6 and Sonnet 4.6 support adaptive thinking where the
model dynamically decides when and how much to think.  This is now
Anthropic's recommended mode and `budget_tokens` is deprecated on these
models.

Add "adaptive" as a valid thinking level:
- Config: `agents.defaults.thinkingDefault: "adaptive"`
- CLI: `/think adaptive` or `/think auto`
- Pi SDK mapping: "adaptive" → "medium" effort at the pi-agent-core
  layer, which the Anthropic provider translates to
  `thinking.type: "adaptive"` with `output_config.effort: "medium"`
- Provider fallbacks: OpenRouter and Google map "adaptive" to their
  respective "medium" equivalents

Closes #30880

Made-with: Cursor

* style(changelog): format changelog with oxfmt

* test(types): fix strict typing in runtime/plugin-context tests

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/agents/model-selection.ts

@@ -14,7 +14,7 @@ export type ModelRef = {
   model: string;
 };
 
-export type ThinkLevel = "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
+export type ThinkLevel = "off" | "minimal" | "low" | "medium" | "high" | "xhigh" | "adaptive";
 
 export type ModelAliasIndex = {
   byAlias: Map<string, { alias: string; ref: ModelRef }>;