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/instance.go

@@ -26,6 +26,7 @@ type AgentInstance struct {
 	MaxIterations             int
 	MaxTokens                 int
 	Temperature               float64
+	ThinkingLevel             ThinkingLevel
 	ContextWindow             int
 	SummarizeMessageThreshold int
 	SummarizeTokenPercent     int
@@ -103,6 +104,12 @@ func NewAgentInstance(
 		temperature = *defaults.Temperature
 	}
 
+	var thinkingLevelStr string
+	if mc, err := cfg.GetModelConfig(model); err == nil {
+		thinkingLevelStr = mc.ThinkingLevel
+	}
+	thinkingLevel := parseThinkingLevel(thinkingLevelStr)
+
 	summarizeMessageThreshold := defaults.SummarizeMessageThreshold
 	if summarizeMessageThreshold == 0 {
 		summarizeMessageThreshold = 20
@@ -169,6 +176,7 @@ func NewAgentInstance(
 		MaxIterations:             maxIter,
 		MaxTokens:                 maxTokens,
 		Temperature:               temperature,
+		ThinkingLevel:             thinkingLevel,
 		ContextWindow:             maxTokens,
 		SummarizeMessageThreshold: summarizeMessageThreshold,
 		SummarizeTokenPercent:     summarizeTokenPercent,