feat(agent): build agent registry

[Svagtlys]

Closes #26

Description

Ship the read-only AgentRegistry — typed, filterable queries over agent_instances ⋈ agents — as the central source of truth for running agents. No new table, no migration: persistence ships with the #25 lifecycle model, and AgentInstanceManager remains the only write path.

Implementation

• RunningAgent frozen read model: instance fields + joined definition fields (agent_name, definition_status, assignments); model_binding deliberately absent per spec.
• AgentRegistry.list_instances — agent_id/session_id/instance-status filters (AND-composed), deterministic created_at, id ordering.
• AgentRegistry.get_instance — raises InstanceNotFoundError on miss (same contract as the manager's _get).
• AgentRegistry.count_by_status — zero-filled per-enum dashboard counter.
• Lenient assignments parsing (warn + empty — reporting surface), while bad status enums fail loud.
• Lifecycle-visibility and never-mutates guarantee tests against the shipped manager; exports from octave.agent (SDK quarantine guard passes).
• Library-level only — no routes (per spec non-goals).

References

• Design doc: .agents/specs/2026-09-27-agent-registry-design.md
• Implementation plan: .agents/specs/2026-09-27-agent-registry.md

Checklist

• Tests passing (368 passed from backend/; +12 new registry tests, no regressions)
• Lint clean (ruff check ., mypy src/octave/agent/registry.py)
• Frontend builds — not applicable (no frontend changes)