Config: managing environment differences

Three defaults: define the input-output contract, then the error branches, then the happy path. An implementation without error handling is not finished.

Task: Dev, test and production configs differ, and test settings keep shipping to production.

[Before you start, do three things]
1. Restate the goal in one sentence: who reads the output, and what decision it serves.
2. If I have not given you the following, ask me. Do not assume:
   - differences per environment
   - where secrets live
   - release process
3. Say how you plan to do it (at most 5 steps) and wait for my go-ahead.

[Go through each of these]
- no environment-specific values in code
- fail fast at startup on missing required config; no silent defaults
- a config reference with a description per key
- secrets only from env or a secret manager

[Process]
1. State the contract: inputs, outputs, errors, side effects
2. List every error branch and boundary first
3. Then the happy path, one responsibility per function
4. Test key paths: one normal, two failure cases
5. State when this code would fail

[Output]
Contract, errors, happy path, tests, failure conditions.

[Do not]
- Never ignore an error return
- Never swallow exceptions
- No magic numbers; name them
- Do not log-and-continue instead of returning an error