feat(observability): add ToolObserver hook for structured tool tracing

[SebastianBoehler]

Summary

Adds a lightweight observer pattern to ToolRegistry that fires after every tool execution. This enables external observability integrations (W&B Weave, OpenTelemetry, custom metrics) without modifying individual tools.

Changes

pkg/tools/registry.go

• Add ToolObserver func type: func(name string, args map[string]interface{}, result *ToolResult, durationMs int64)
• Add observer field to ToolRegistry
• Add SetObserver() method
• Fire observer in ExecuteWithContext after every tool call

pkg/agent/loop.go

• Add SetToolObserver() to AgentLoop — propagates observer to all agent tool registries

cmd/picoclaw/main.go

• Wire observer in agentCmd when PICOCLAW_WEAVE_OBSERVE=1
• Emits WEAVE_TOOL_EVENT: JSON lines to stderr for downstream consumption

Design

The observer is a zero-cost abstraction when not set — a single nil check before the call. No allocations, no goroutines.

The WEAVE_TOOL_EVENT: line format emitted to stderr:

WEAVE_TOOL_EVENT:{"tool":"exec","duration_ms":42,"is_error":false,"args":{...}}

This is intentionally transport-agnostic — any process reading the agent's stderr can parse these lines. The W&B Weave integration reads them from the agent subprocess and logs structured traces per run, but the hook itself has no Weave dependency.

Usage

// Custom observer — log to any backend
agentLoop.SetToolObserver(func(name string, args map[string]interface{}, result *tools.ToolResult, durationMs int64) {
    myTracer.RecordToolCall(name, durationMs, result.IsError)
})

Or via env var for the built-in stderr emitter:

PICOCLAW_WEAVE_OBSERVE=1 picoclaw agent -m "your prompt"

[nikolasdehor]

The observer pattern is well-designed — zero-cost nil check, thread-safe via RLock, clean API. Two notes:

1. Unrelated changes: Removes OneBot transcriber wiring and a duplicate Name field. These should be separate commits or documented in the PR description.

2. Fragile JSON construction: The event JSON is built via fmt.Sprintf with %q and raw %s. If argsJSON is malformed, the outer JSON breaks. Safer to use json.Marshal for the entire event struct.

LGTM for the core observer pattern.

[lxowalle]

1. Please submit only one feature per PR.
2. The http_fetch and web_fetch tools conflict.

[SebastianBoehler]

Removed conflicting features

[CLAassistant]

All committers have signed the CLA.