feat: add extended thinking support for Anthropic models (#1076)

* feat: add extended thinking support for Anthropic models

Support configurable thinking levels (off/low/medium/high/xhigh/adaptive)
via `agents.defaults.thinking_level` config field.

- "adaptive": uses Anthropic's adaptive thinking API (Claude 4.6+)
- "low/medium/high/xhigh": uses budget_tokens (all thinking-capable models)
- "off": disables thinking (default)

API constraints handled:
- Temperature cleared when thinking is enabled
- budget_tokens clamped to max_tokens-1
- Thinking response blocks parsed into Reasoning field

Relates to #645, #966

* fix: address PR review feedback for thinking support

- Add ThinkingCapable interface for provider capability detection
- Warn when thinking_level is set but provider doesn't support it
- Warn when temperature is cleared due to thinking enabled
- Adjust budget values per Anthropic best practices (medium=16K, xhigh=64K)
- Add budget clamp warning and 80% threshold warning
- Add parseResponse thinking block tests
- Add thinking_level field to config.example.json

* refactor: move ThinkingLevel from AgentDefaults to ModelConfig

Thinking is a model-level capability, not a global agent property.
Per-model config avoids silent ignoring on non-Anthropic providers
and eliminates spurious warning logs in multi-provider setups.

Addresses PR #1076 review feedback from @yinwm.

pkg/agent/thinking.go

@@ -0,0 +1,39 @@
+package agent
+
+import "strings"
+
+// ThinkingLevel controls how the provider sends thinking parameters.
+//
+//   - "adaptive": sends {thinking: {type: "adaptive"}} + output_config.effort (Claude 4.6+)
+//   - "low"/"medium"/"high"/"xhigh": sends {thinking: {type: "enabled", budget_tokens: N}} (all models)
+//   - "off": disables thinking
+type ThinkingLevel string
+
+const (
+	ThinkingOff      ThinkingLevel = "off"
+	ThinkingLow      ThinkingLevel = "low"
+	ThinkingMedium   ThinkingLevel = "medium"
+	ThinkingHigh     ThinkingLevel = "high"
+	ThinkingXHigh    ThinkingLevel = "xhigh"
+	ThinkingAdaptive ThinkingLevel = "adaptive"
+)
+
+// parseThinkingLevel normalizes a config string to a ThinkingLevel.
+// Case-insensitive and whitespace-tolerant for user-facing config values.
+// Returns ThinkingOff for unknown or empty values.
+func parseThinkingLevel(level string) ThinkingLevel {
+	switch strings.ToLower(strings.TrimSpace(level)) {
+	case "adaptive":
+		return ThinkingAdaptive
+	case "low":
+		return ThinkingLow
+	case "medium":
+		return ThinkingMedium
+	case "high":
+		return ThinkingHigh
+	case "xhigh":
+		return ThinkingXHigh
+	default:
+		return ThinkingOff
+	}
+}