feat: implement Agent Teams executor backend with hook integration

[michael-wojcik]

Context

Deferred from PACT v3 Phase D. Tracked for v3.1+. Combines two related items: the Agent Teams executor backend and TeammateIdle/TaskCompleted hook integration.

Description

Implement a real Agent Teams executor backend for scoped orchestration, replacing the current rePACT-based executor. This would use Claude Code's Agent Teams feature (TeamCreate, SendMessage, Task with team_name) to execute sub-scopes as actual teammates rather than sequential rePACT cycles.

Includes:

• Executor backend: Wire up the executor interface (currently documented in pact-scope-contract.md) to use Agent Teams tools
• Hook integration: Implement TeammateIdle and TaskCompleted hooks to coordinate teammate lifecycle
• Feature gate: Gate behind experimental flag since Agent Teams is still experimental

Current State

• Agent Teams API is experimental, gated behind CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1
• v2.8.0 documents the API mapping (TeamCreate, SendMessage, TeamDelete) in pact-scope-contract.md but does not implement it
• The executor interface is backend-agnostic by design — rePACT is the current executor, Agent Teams would be an alternative
• No consumers exist for TeammateIdle/TaskCompleted hooks without an Agent Teams executor

Blockers

• Agent Teams must reach stable release (or at least mature significantly)
• Real-world scoped orchestration usage needed to validate the approach

Priority

Medium — depends on Agent Teams API stability.

References

• PR feat: Phase D revised — memory rationalization, agent persistent memory, expanded verification (v2.8.0) #156 (Phase D revised, v2.8.0) — documentation update
• pact-plugin/protocols/pact-scope-contract.md (executor interface + API mapping)
• pact-plugin/protocols/pact-scope-phases.md (ATOMIZE/CONSOLIDATE phases)
• docs/agent-teams-feature/preparation/agent-teams-research.md (research)