Elixir Context Fn

Add a context function

Read portal/AGENTS.md and sections 1, 2, 4, 5, and 7 of portal/.agent/kb/rules/elixir-layered-contexts.md as relevant to the operation. Read the target context and match its existing functions.

For Subject-gated public APIs, Subject is the last required positional argument. Check permission before database access, scope with Authorizer.for_subject immediately before the Repo call, and return the documented tagged tuple. Read section 1.2 for self-service and section 1.4 for already-authorized internal helpers; do not apply a generic template over those explicit exceptions.

Choose the existing Repo operation for the contract: fetch for a tagged row, list for a paginated result, insert for creation, fetch_and_update for guarded single-row mutations, and Multi for multi-row composition. Confirm signatures in source. Queries own predicates; changesets own casts, validations, and state transitions.

Audit writes belong in the transaction; external side effects belong after the outer commit. Use named per-event broadcast helpers. Do not place audit insertion inside an after_commit callback or nest after_commit under another transaction.

Reuse permission accessors; introduce a new one only for a new capability and wire its role grants. State the function's contract in its documentation. Cover happy, denial, and cross-account behavior for both reads and writes. Use focused checks while working; the lead owns ./run gate portal --changed, the relevant trust-boundary review, and the complete ./run gate portal before pushing.