Add in-process local runtime and dual-runtime examples with Task automation

[vinodvx]

Description

• Add in-process runtime — zero setup, no Temporal required; full feature parity (tools, MCP, A2A, sub-agents, streaming, AG-UI, approvals, conversation, observability)
• Sub-agents run recursively in-process with all events fanning in to the parent stream automatically
• Refactor sub-agent routing to pluggable per-runtime design — each runtime owns its own route structures; pkg/agent is runtime-agnostic
• Refactor examples to support both runtimes via AGENT_RUNTIME=local (default, zero setup) / AGENT_RUNTIME=temporal
• Add Task-driven examples automation to run the suite on local and Temporal, with combined runs and pass/fail reports
• Update docs — README and CONTRIBUTING for dual-runtime support

Test plan

• [*] go test ./... passes
• [*] In-process examples run without Temporal server
• [*] Temporal examples run with Temporal server
• [*] Sub-agent delegation and event fan-in verified on both runtimes
• [*] Depth limiting enforced on both runtimes
• [*] task examples:local, task examples:temporal, and task examples:all complete with reports

Type of change

• Bug fix (non-breaking change which fixes an issue)
• [*] New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• [*] Documentation update
• [*] Refactor / chore

Related issues

Close #12

Checklist

• [*] I have run make lint and make test locally
• [*] I have run make tidy if I added or removed dependencies
• [*] Commit messages follow conventional commits (e.g. feat:, fix:, docs:)
• [*] I have added/updated tests for my changes
• [*] Documentation is updated if needed