Add observability across build_context/consolidate/search chains

[Sunnylincc]

Motivation

• Provide unified, structured observability for the three main flows (build_context, consolidate, search) so agent/session/request context and key metrics are consistently available.
• Make telemetry pluggable with a safe default to allow easy integration with Prometheus/OpenTelemetry later while keeping tests simple.

Description

• Add a JSON log formatter and configure_json_logger helper in hippocortex/observability.py that emits agent_id, session_id, and request_id as structured fields.
• Introduce a telemetry abstraction in hippocortex/telemetry.py (Telemetry protocol) with NoOpTelemetry default and InMemoryTelemetry for tests, and wire it into the SDK via a telemetry field on HippoCortex.
• Instrument hippocortex/hippo/episodic_store.py event writes to increment hippocortex.events.write and emit a structured event_written log entry, and instrument HippoCortex.consolidate and CortexAPI.search to observe hippocortex.consolidation.duration_ms, hippocortex.search.duration_ms, and hippocortex.search.hit_rate and emit structured logs.
• Extend the router decision (RoutingDecision) with an explain payload containing matched keywords, scores, and budget allocation and log the router decision from build_context.
• Add tests/test_observability.py to assert that router explain is present, structured logs include agent_id/session_id/request_id, and telemetry metrics are emitted via InMemoryTelemetry.

Testing

• Ran the focused test set: pytest -q tests/test_observability.py tests/test_router_policy.py tests/test_working_memory.py tests/test_consolidation.py and verified all tests passed.
• Ran the full test suite with pytest -q and confirmed 17 passed.
• Tests specifically assert presence of structured log fields, RoutingDecision.explain contents, and that telemetry counters/observations include hippocortex.events.write, hippocortex.consolidation.duration_ms, hippocortex.search.duration_ms, and hippocortex.search.hit_rate.

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Codex Task