feat: add Explore agent with PydanticAI tool calling

Implements the first Claude-like agent for codebase exploration:

Core Features:
- Explore agent with glob, grep, read, and bash tools
- Native PydanticAI tool calling with Ollama/Mistral Nemo
- Sanitized Ollama provider (fixes content:null issue)
- REST API endpoints for agent execution

Tool Infrastructure:
- BaseTool abstract class with ToolResult dataclass
- ReadFileTool, GlobFilesTool, GrepContentTool, BashReadOnlyTool
- Path validation and sandboxing support

CLI Client (separate package for future extraction):
- webber-cli command with chat, explore, status commands
- Communicates with Webber API backend
- Rich console output with theming

Configuration:
- Dev server on port 8095 (production uses 8086)
- Mistral Nemo optimizations (temp 0.3, tool_choice required)

Tests: 24 tests covering tools and API endpoints

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/mistral-instructions.md

@@ -0,0 +1,14 @@
+## Background
+
+Research with Gemini identified key issues with mistral-nemo and tool calling:
+- "Pre-computation Hallucination" - model answers before using tools
+- High default temperature (0.7-0.8) causes wandering
+- Model is "chatty and confident" - needs explicit constraints
+
+## Key Recommendations from Gemini Research
+
+1. **Temperature 0.0** for tool-calling agents (deterministic, follows schema)
+2. **Chain of Thought (CoT)** - force step-by-step reasoning
+3. **Negative constraints** - tell model what NOT to do (Nemo responds better)
+4. **Explicit tool descriptions** - verbose docstrings with "never estimate yourself"
+5. **"Strictly tool-based assistant"** pattern - NO internal knowledge claim