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/auto-reply/thinking.test.ts

@@ -33,6 +33,12 @@ describe("normalizeThinkLevel", () => {
   it("accepts on as low", () => {
     expect(normalizeThinkLevel("on")).toBe("low");
   });
+
+  it("accepts adaptive and auto aliases", () => {
+    expect(normalizeThinkLevel("adaptive")).toBe("adaptive");
+    expect(normalizeThinkLevel("auto")).toBe("adaptive");
+    expect(normalizeThinkLevel("Adaptive")).toBe("adaptive");
+  });
 });
 
 describe("listThinkingLevels", () => {
@@ -54,6 +60,11 @@ describe("listThinkingLevels", () => {
   it("excludes xhigh for non-codex models", () => {
     expect(listThinkingLevels(undefined, "gpt-4.1-mini")).not.toContain("xhigh");
   });
+
+  it("always includes adaptive", () => {
+    expect(listThinkingLevels(undefined, "gpt-4.1-mini")).toContain("adaptive");
+    expect(listThinkingLevels("anthropic", "claude-opus-4-6")).toContain("adaptive");
+  });
 });
 
 describe("listThinkingLevelLabels", () => {