Secrets store: sensitive:false plaintext rows (readable non-secret config values)

Follow-up to #715 / PR #1554 (store) and #1550 / PR #1582 (consumption), from the P1b design decision.

## Problem

The `hdb_secret` store treats every row as sensitive: values are write-only through the operations API (never returned by reads), redacted in MCP audit surfaces, and `read_audit_log` is blocked for the table. That is the right default, but some configuration values are *shared, non-secret* settings (feature flags, service URLs, tuning values) that teams still want centrally stored, replicated, and delivered through the same `env:` declaration surface — without losing the ability to read them back in Studio/ops tooling.

Today the only options for non-secret values are inline literals in each component's `env:` block (not centrally managed) or storing them as secrets (write-only, so ops can't view/diff them).

## Proposal

Add a `sensitive: false` flag on `hdb_secret` rows (default `true`, preserving current behavior):

- `sensitive: false` rows store the value as plaintext (no `enc:` envelope required) and **are readable back** through the operations API (`get_secret`/`list_secrets` return the value) and Studio.
- Redaction, audit-read blocking, and MCP default-deny continue to apply only to `sensitive: true` rows — audit redaction becomes conditional on the flag.
- Consumption side (#1582's `env:` declarations, grants/two-tier delivery) is unchanged: a declaration resolves the row the same way regardless of sensitivity.
- Flag flips: `false → true` is allowed (tightening). `true → false` must be rejected — the stored value was written under write-only expectations and must not become readable after the fact; require a new value write to downgrade.

## Acceptance

- Row schema + operations validation accept `sensitive: false`; default remains `true`.
- Non-sensitive rows readable via ops API; sensitive rows remain write-only (existing tests unchanged).
- `true → false` transition rejected without a new value.
- Audit/MCP redaction conditional on the flag, with tests for both settings.