fix: stop redacting the model provider's name from masked output

[brahyam]

createSecretValueMasker treats every env value of eight characters or more
as a secret to redact. A provider env that carries MODEL_PROVIDER therefore
has the literal words openrouter (10) and anthropic (9) replaced with
<redacted:MODEL_PROVIDER> everywhere they occur in the masked text —
including in prose that has nothing to do with the variable.

MODEL_PROVIDER names a provider; it is never a credential.
NON_SECRET_ENV_KEYS already carries BROWSE_LAB_MODEL_PROVIDER for exactly
this reason, so this declares the plain one beside it.

The existing "plumbing keys and short values are not masked" case gains the
key rather than a new case being added beside it — that test is already the
one place asserting the allowlist is honoured, and its command string now
contains the provider name, so the case goes red if the entry is dropped
again. Verified by reverting the one-line change and watching it fail: an
allowlist entry no test can distinguish from its own absence is not pinned by
anything.

test/secret-masking.test.ts and the masker's callers
(test/memory-provider-config.test.ts, test/memorable-capture.test.ts,
test/memorable-relay.test.ts) pass: 42 tests. npm run typecheck, eslint and
prettier --check pass.

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