Secrets hardening: context-bound envelopes (enc:v2), key rotation walk, audit stream

Hardening phase (5) of the secrets-management plan, after the store (#715), consumption, and custody (harper-pro#166) land.

- `enc:v2`: AAD-bind ciphertext to its context (cluster | secret name) so a transplanted envelope fails to decrypt.
- Rotation: operator-triggered re-encrypt walk over hdb_secret keyed on envelope `kid`; custody layer holds a kid→key set during the walk so old ciphertext keeps decrypting until re-encrypted.
- First-class audit stream for secret writes/grants/reads (component-granularity for reads) — who/what/when, never values.
- External KMS/vault custody provider (#816) belongs to this phase.

🤖 Drafted by Claude (Fable 5) on Kris's behalf.