Summary

• Add GeminiThinkingLevel enum (Minimal/Low/Medium/High, lowercase serde matching Gemini API) and GeminiThinkingConfig struct (thinkingLevel/thinkingBudget/includeThoughts, camelCase) to zeph-llm
• GeminiProvider gains builder methods with_thinking_level(), with_thinking_budget() (fallible, validates -1/0/1..=32768), with_include_thoughts()
• GeminiConfig in zeph-core gains thinking_level, thinking_budget, include_thoughts optional fields
• Thinking config wired at all three construction sites (primary, orchestrator, router)
• --init wizard adds thinking_level select prompt in the Gemini section
• Covers both Gemini 3+ (thinkingLevel string) and Gemini 2.5 (thinkingBudget integer) APIs

Configuration

[llm.gemini]
model = "gemini-2.5-flash-thinking"
thinking_level = "medium"   # minimal | low | medium | high (Gemini 3+)
thinking_budget = 1024      # -1=dynamic, 0=off, 1-32768 (Gemini 2.5)
include_thoughts = false    # include thinking summaries in response

Test plan

• ThinkingLevel serializes to correct lowercase strings (all 4 variants round-trip)
• GeminiThinkingConfig serializes with camelCase field names
• skip_serializing_if omits thinking_config from JSON when all fields are None
• Builder methods wire through to make_gen_config()
• with_thinking_budget() rejects out-of-range values (-2, 32769) with LlmError
• Budget edge values 0 (disable) and -1 (dynamic) pass through correctly
• TOML config parse tests for GeminiConfig.thinking_level/thinking_budget in zeph-core
• 5366 tests pass, 0 failures

Closes #1652.