Let the agents AppRole mint unkin-agent Gitea tokens (#114)

Why: AI coding agents authenticate to Gitea as Ben using Ben's token. With the unkin-agent identity now in place (terraform-git PR #59), the agents AppRole should issue that account's tokens directly so agent commits and PRs are attributable and carry only least-privilege scopes.

How:
- add a gitea secrets-engine role minting ephemeral tokens for unkin-agent scoped to write:repository, write:issue, read:user — push branches and open PRs, never merge or administer
- add a policy granting read on gitea/creds/unkin-agent, bound to the agents AppRole, mirroring the agent-* Kubernetes creds bindings

Depends on terraform-git PR #59: the unkin-agent Gitea account must exist before minted tokens work. The vault-plugin-secrets-gitea engine is already live (plugin v0.1.0 registered, gitea mount configured), so no engine/plugin change is needed here.

Reviewed-on: #114
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

config/gitea_secret_backend_role/gitea/unkin-agent.yaml

@@ -0,0 +1,20 @@
+# Role minting ephemeral tokens for the unkin-agent bot user -- the shared
+# identity Ben's AI coding agents use to submit work. The agent clones/pushes
+# code and opens pull requests, so it gets write on repositories (clone + push +
+# PR create) and write on issues (PR/issue comments). Read is implied by write.
+# No admin/org/user-write scopes, so it can never merge via API privilege; merge
+# is blocked separately by branch protection (merge whitelist = Owners).
+# read:user is required because tea (and most API clients) validate the login
+# via GET /api/v1/user, which 403s without it.
+# Reading gitea/creds/unkin-agent mints a lease-bound token deleted from Gitea
+# on revoke/expiry. Consumed by the "agents" AppRole (see
+# policies/gitea/creds/unkin-agent.yaml).
+---
+username: unkin-agent
+scopes:
+  - write:repository
+  - write:issue
+  - read:user
+token_name_prefix: vault-unkin-agent
+ttl: 3600      # 1h
+max_ttl: 14400 # 4h