docs: document set_config transaction-local gotcha in pgsql-test

[pyramation]

Summary

Adds documentation about the transaction-local behavior of set_config(..., true) in two places:

postgres/pgsql-test/README.md

• Added a warning callout in the "Role-Based Context" section explaining that setContext() uses set_config(..., true) which is transaction-local
• Inside it() blocks this works automatically (transaction started by beforeEach)
• In beforeAll(), context is lost between queries — must wrap in explicit db.begin()/db.commit()
• Includes code example showing the correct pattern

postgres/pgsql-test/AGENTS.md

• Added a new "Transaction-Local Context" section with the same key information in concise format for AI agents

Review & Testing Checklist for Human

• Verify the explanation matches the actual implementation in pgsql-client/src/context-utils.ts (set_config with true for is_local)

Notes

Documentation-only change. Companion PRs going to constructive-db and constructive-skills repos.

Link to Devin session: https://app.devin.ai/sessions/ef4c0849684f44b4bb4dd83074c27d48
Requested by: @pyramation

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