feat: add runtime registries for tools, MCP, A2A, and sub-agents with per-run resolution

[vinodvx]

Description

Introduces runtime registries for tools, MCP, A2A, and sub-agents. Capabilities are resolved per run (Run, Stream, RunAsync) from the current registry state, so callers can add or remove tools, MCP/A2A clients, and sub-agents after NewAgent without restarting the agent.

Changes

• Registries — ToolRegistry, MCPRegistry, A2ARegistry, and SubAgentRegistry with Register / Unregister; exposed on Agent via ToolRegistry(), MCPRegistry(), A2ARegistry(), and SubAgentRegistry(). Sugar options (WithTools, WithMCPConfig, WithSubAgents, etc.) still work and merge at build time.
• Per-run resolution — Each Run, Stream, and RunAsync builds the tool list and sub-agent specs from the live registries at call time.
• Runtime — Local and Temporal runtimes receive the resolved tool list on each execute request. Temporal uses fetchTools and fingerprint verification against the worker’s current registry state.
• RunAsync — Returns (<-chan AgentRunAsyncResult, error); approvals use WithApprovalHandler (same as Run).
• ToolKind — Optional interfaces.ToolKindProvider and interfaces.KindOf(); MCP, A2A, sub-agent, and retriever tools report their kind ("mcp", "a2a", "sub-agent", "retriever").
• Docs & examples — README section Managing Capabilities at Runtime; examples/agent_with_tools/dynamic_registry example.
• Cleanup — Removed pkg/tools/registry in favor of pkg/agent registries; tests and example updates.

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

Closed #23

Checklist

• I have run make check
• I have run task examples:all
• 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

[codecov]

Codecov Report

❌ Patch coverage is 74.34842% with 187 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pkg/agent/config.go	81.36%	22 Missing and 19 partials ⚠️
pkg/agent/agent.go	59.59%	29 Missing and 11 partials ⚠️
internal/runtime/temporal/agent_workflow.go	33.33%	18 Missing and 6 partials ⚠️
pkg/agent/a2a_registry.go	72.72%	12 Missing and 9 partials ⚠️
pkg/agent/mcp_registry.go	74.02%	10 Missing and 10 partials ⚠️
pkg/agent/tool_registry.go	83.63%	5 Missing and 4 partials ⚠️
pkg/agent/subagent_registry.go	82.22%	4 Missing and 4 partials ⚠️
internal/runtime/temporal/runtime.go	77.41%	6 Missing and 1 partial ⚠️
benchmarks/setup/mock_tool.go	0.00%	4 Missing ⚠️
internal/runtime/local/agent_loop.go	76.47%	3 Missing and 1 partial ⚠️
... and 5 more

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