secrets: a managed credential can be readonly, plus review fixes

- SourceBlock -> SecretSource, SourceEntries -> SecretEntries
- readonly is legal on a managed env entry: the fetch writes the
  record, not through the shell's assignment gate, so a credential the
  agent cannot overwrite is still filled
- one example per language; secrets_op is gone
- resolveValue recurses only into plain objects, so a config field
  holding a class instance keeps its prototype
- TS clis.*.config, python's no-secrets-block walk, and the clone and
  load override mounts all resolve pointers now
- every refusal logs the exception kind, never the source's own words