[Epic] Secrets & dynamic env-var management

Tracking epic for secrets/env-var distribution design: Key Vault as a secrets source, the hdb_secret system table/grants design, and broadcasting process.setEnv() changes to workers.

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_Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass._

Start by separating the epic into its three named areas: Key Vault as a secrets source, the hdb_secret system table and grants design, and broadcasting process.setEnv() changes to workers. Read the related project entry points and document the intended design and acceptance criteria for each area; done means the scope is decomposed into actionable, testable work.

Written by the indexing model from the issue text.