feat(ws): add polling fallback rollout controls

[iAziz786]

Summary

Add independent rollout flags, effective delivery gating via hello handshake, polling pause/resume coordination, and duplicate-execution safety during polling/WS overlap transitions.

Changes

Hostlink

• Config: HOSTLINK_WS_RESULTS_ENABLED, HOSTLINK_WS_DELIVERY_ENABLED, HOSTLINK_WS_POLLING_FALLBACK_THRESHOLD, plus parseBoolEnabled helper
• Rollout coordinator: shared rollout.Coordinator tracks effective delivery state, drives polling gate and timeout-based fallback
• Protocol: agent.hello.payload.capabilities (results_enabled, delivery_enabled), agent.hello_ack.payload.delivery_enabled with reconciliation directives
• WS client gating: delivery/reject task.deliver when disabled, skip result channel when results disabled, preserve HTTP fallback
• Runner: PollingGate interface suppresses fetch when coordinator says polling not needed

Rails

• Rollout policy: AgentWebsocketRolloutPolicy with env-backed gateway/delivery enablement + optional canary allowlists by agent/org PID
• Hello activation: computes effective delivery_enabled from hostlink capabilities + server policy, includes in agent.hello_ack
• Registry: tracks per-agent delivery state alongside connectivity; post-commit delivery checks respect it
• Polling fallback: AgentsController#index returns same-agent leased pending tasks with execution_attempt_id; foreign active leases hidden

Duplicate Safety

• Runner dedupes by task_id + execution_attempt_id, plus task_id fallback during polling/WS overlap windows
• Post-hello delivery check and after-commit triggers both gate on effective delivery state

Docs

• Updated SPEC.md, specs/10-polling-fallback-rollout.md, .context/agents-and-heartbeats.md with actual flag names and fallback rules