feat(store): inject implicit graph-linking metadata in search results via topic_key siblings

[Snakeblack]

Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

Problem Description / Descripción del Problema

English: When mem_search returns results, each observation is isolated — the agent has no awareness of related observations that share the same topic_key. For example, if an agent finds 'JWT Middleware' (topic: auth/jwt), it doesn't know about 'Token Rotation Bug' or 'Session Fix' that share the same topic. This forces additional manual searches to build relational context.

Español: Cuando mem_search retorna resultados, cada observación está aislada — el agente no tiene awareness de observaciones relacionadas que comparten el mismo topic_key. Esto fuerza búsquedas manuales adicionales para construir contexto relacional.

Proposed Solution / Solución Propuesta

English: Post-process search results in two layers:

1. Store layer: New RelatedByTopicKeys() function that queries siblings by topic_key using the existing idx_obs_topic composite index (efficient, no schema migration)
2. MCP layer: Inject related metadata as enriched text in handleSearch response:
   Related (topic 'auth/jwt'): ID 4 (JWT Middleware), ID 12 (Token Rotation Bug)

Limits: max 5 siblings per topic_key. Excludes soft-deleted observations. Filters by project + scope. No query executed when results have no topic_keys.

Español: Post-procesamiento en dos capas: (1) nueva función RelatedByTopicKeys() en store usando el índice compuesto idx_obs_topic existente, y (2) inyección de metadata relacional como texto enriquecido en handleSearch. Máximo 5 hermanos por topic_key, filtra por project+scope, excluye soft-deleted.

Affected Area

Store (new query function), MCP Server (search response enrichment)

Additional Context

• Leverages existing idx_obs_topic(topic_key, project, scope, updated_at DESC) index
• No schema migration needed
• New RelatedObservation struct: {ID int64, Title string}
• Separation of concerns: store returns pure data, handler serializes to text