Add agentvault seed-oauth for oauth2-proxy credentials

Seeding an oauth2-proxy secret by hand means an agent shell-plumbing a
client secret and a cookie secret, which the classifier blocks. seed-oauth
does it in one self-contained invocation: it reads the KV path, fills in
only the keys that are missing, preserves everything else and prints key
names and the new version, never a value.

- Add SeedOAuth in internal/agent: read-modify-write of the client_id,
  client_secret and cookie_secret keys with per-key created/kept/rotated
  actions and a no-op when nothing changed.
- Generate secrets from 32 crypto/rand bytes; cookie_secret is base64url so
  it decodes to exactly the 32 bytes oauth2-proxy requires.
- Add ReadKVOptional (missing secret = empty) and WriteKVAny (non-string
  fields survive a round trip) to the KV-v2 client.
- Wire the seed-oauth subcommand and document it in README and AGENTS.md.
- Cover fresh create, patch-preserves-client_secret, other-key
  preservation, --rotate, idempotence, denial errors and secret leakage.

README.md

@@ -162,6 +162,42 @@ Errors name the failing stage: AppRole login, KV read denied (policy not
 applied), outpost not found (terraform not applied), `view_key` failure, or KV
 write denied.
 
+### seed-oauth
+
+Make a Vault KV-v2 path hold a complete oauth2-proxy credential set. It is a
+read-modify-write: `client_id` is set from the flag, `client_secret` and
+`cookie_secret` are generated (32 bytes from `crypto/rand`) only when missing,
+every other key on the path is written back untouched, and nothing is written
+at all when the secret is already correct. `cookie_secret` is base64url so it
+decodes to exactly the 32 bytes oauth2-proxy requires.
+
+```bash
+agentvault seed-oauth \
+  --path kubernetes/namespace/repospawner/default/oauth-credentials \
+  --client-id 4f1c…
+```
+
+```
+path:          kv/kubernetes/namespace/repospawner/default/oauth-credentials
+keys:          client_id, client_secret, cookie_secret
+  client_id:     created
+  client_secret: kept
+  cookie_secret: created
+version:       4
+```
+
+That is the common case: the provider's `client_secret` already lives on the
+path, so only the missing keys are added. A run with nothing to do prints
+`version:       unchanged` and issues no write.
+
+Flags: `--path` and `--client-id` are required; `--kv-mount` (default `kv`) and
+`--rotate` override the rest. `--rotate` regenerates both secrets — only use it
+when the IdP provider's secret is being rotated alongside, since a rotated
+`client_secret` no longer matches the provider.
+
+Errors name the failing stage: AppRole login, KV read denied, or KV write
+denied. Only key names, actions and the KV version are printed.
+
 ## Build & package
 
 ```bash