Add structured logging infrastructure

[Furisto]

Implement comprehensive structured logging using slog across agent runtime, task reconciliation, and model providers with:

• New logging layer: agent/logger.go with standardized log key constants
• Context propagation: agent/context.go for task/agent/model tracking
• Model providers: OpenAI, Anthropic, Gemini with operation telemetry
• Task reconciliation: Detailed logging of reconciliation phases and tool execution
• Filesystem tools: Logging for file operations and search results
• Code interpreter: Execution timing and output tracking
• Runtime lifecycle: Component initialization and shutdown logging

Key features:

• Centralized log key constants for consistency (KeyTaskID, KeyModel, etc.)
• Operation timing with LogOperationStart/End helpers
• Token usage logging with cache hit ratio calculations
• Retryable error logging with retry-after timing

This enables better observability into task execution, model invocations, and tool operations for debugging and performance monitoring.

[coderabbitai]

Walkthrough

This PR adds structured logging with Go's slog across the backend: a new agent logger utility, integration into the Runtime and TaskReconciler, and extensive logging in model providers and filesystem/interpreter tools. It introduces timing and token usage metrics and replaces many ad-hoc logs with structured helpers. A few public APIs/structs were changed: ModelProviderFactory.CreateClient signature, RuntimeOptions (LoggerConfig) and WithLoggerConfig, Runtime (logger field), and TaskReconciler.publishError signature. Functional behavior is otherwise unchanged.

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📥 Commits

Reviewing files that changed from the base of the PR and between 96f836f and 6377e1f.

📒 Files selected for processing (13)

• backend/agent/client.go (2 hunks)
• backend/agent/logger.go (1 hunks)
• backend/agent/runtime.go (6 hunks)
• backend/agent/task_reconciler.go (20 hunks)
• backend/model/anthropic_provider.go (7 hunks)
• backend/model/gemini.go (7 hunks)
• backend/model/openai_completion.go (6 hunks)
• backend/tool/codeact/interpreter.go (4 hunks)
• backend/tool/filesystem/create.go (3 hunks)
• backend/tool/filesystem/find.go (5 hunks)
• backend/tool/filesystem/grep.go (3 hunks)
• backend/tool/filesystem/list.go (4 hunks)
• backend/tool/filesystem/read.go (3 hunks)

🚧 Files skipped from review as they are similar to previous changes (5)

• backend/tool/filesystem/create.go
• backend/tool/filesystem/grep.go
• backend/model/gemini.go
• backend/tool/filesystem/find.go
• backend/tool/filesystem/read.go

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