feat: support parallel tool calling

[renezander030]

Summary

• When ParallelToolCalls is enabled in AgentConfig and the LLM returns 2+ tool calls, execute them concurrently via errgroup
• Results are collected in original order for deterministic observe-phase behavior
• Sequential execution remains the default; parallel mode is opt-in per agent configuration

Changes

• internal/types/agent.go: add ParallelToolCalls bool config field
• internal/agent/act.go: refactor executeToolCalls into composable helpers (runToolCall, executeSingleToolCall, executeToolCallsParallel) for parallel support and testability

Design decisions

• Best-effort strategy: a failing tool does not cancel siblings, partial results are collected
• Event emission (tool-hint) happens inside each goroutine for real-time UI progress; result/exec events are emitted in original order after all goroutines complete
• Uses golang.org/x/sync/errgroup (already in go.mod)
• No behavioral change when ParallelToolCalls is false (default)

Test plan

• Sequential mode: verify identical behavior with ParallelToolCalls: false
• Parallel mode: configure 2+ independent tools, verify concurrent execution and correct result ordering
• Failure handling: one tool timeout/error should not affect other tool results

Fixes #707