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.

AGENTS.md

@@ -31,7 +31,7 @@ parsing, watch-state comparison, git worktree helpers).
 cmd/agentpr/main.go      # agentpr CLI (pr create / pr comment / whoami)
 cmd/watchpr/main.go      # watchpr CLI (poll + meaningful-change exit)
 cmd/agentws/main.go      # agentws CLI (new / list / rm / clean / token / credential)
-cmd/agentvault/main.go   # agentvault CLI (seed-outpost)
+cmd/agentvault/main.go   # agentvault CLI (seed-outpost / seed-oauth)
 internal/agent/          # shared plumbing:
   token.go               #   env config + in-process Gitea-token cache
   vault.go               #   AppRole login + read the gitea creds path
@@ -42,6 +42,7 @@ internal/agent/          # shared plumbing:
   vaultkv.go             #   AppRole-authenticated Vault client + KV-v2 read/write
   authentik.go           #   Authentik REST client (outpost search, token view_key)
   seedoutpost.go         #   seed-outpost flow (Authentik token -> Vault KV)
+  seedoauth.go           #   seed-oauth flow (oauth2-proxy credential set in Vault KV)
 go.mod                   # module git.unkin.net/unkin/agent-tools
 Makefile                 # build / test / lint / completions / rpm / version-bump
 packaging/nfpm.yaml      # nfpm spec (envsubst-templated) for the RPM (all binaries)
@@ -148,12 +149,37 @@ printed. Errors are wrapped per stage (login / read denied / outpost missing /
 view_key / write denied) via the `ErrVaultDenied`, `ErrVaultNotFound` and
 `ErrOutpostNotFound` sentinels.
 
+## agentvault seed-oauth
+
+`agentvault seed-oauth --path <kv/path> --client-id <id>` makes a KV-v2 path
+hold a complete oauth2-proxy credential set, in-process:
+
+1. AppRole login (shared `approleLogin`), then a KV-v2 read via
+   `ReadKVOptional` — a 404 or a deleted version means "empty", not an error,
+   so the first seed of a path works.
+2. Desired keys are computed over the existing map: `client_id` from the flag
+   (`kept`/`created`/`updated`), `client_secret` and `cookie_secret` generated
+   from 32 `crypto/rand` bytes only when absent or when `--rotate` is set
+   (`kept`/`created`/`rotated`). `cookie_secret` is base64url so it decodes to
+   exactly the 32 bytes oauth2-proxy demands; `client_secret` is standard
+   base64.
+3. Any other key on the path is carried through unchanged (`preserved`), which
+   is why the write goes through `WriteKVAny` rather than `WriteKV`.
+4. The write is skipped entirely when nothing changed; the command then prints
+   `version: unchanged`.
+
+Only key names, per-key actions and the new KV version are printed. Errors are
+wrapped per stage (login / read denied / write denied) via `ErrVaultDenied`.
+
 ## Gotchas
 
 - `watchpr` exits 0 with no output changes on `--once` (just prints state).
 - The token cache is process-wide (`sync.Once`); tests call the unexported
   `fetchGiteaToken` to avoid it.
 - `agentvault` never puts a secret in an error string: Vault decode failures and
-  Authentik `view_key` responses are reported without their bodies.
+  Authentik `view_key` responses are reported without their bodies, and
+  `seed-oauth` reports key names only.
+- `--rotate` regenerates the `client_secret` too, which then no longer matches
+  the IdP provider unless that is rotated alongside.
 - CI "combined status" comes from `/commits/{sha}/status`; an empty head SHA
   yields an empty state without an API call.