feat: add bao-secrets composite action (public, no secrets) (#545)

Shared actions live in a PUBLIC repo so CI runners can clone them anonymously.
They can't clone the private spikersoft-infrastructure repo (a job token is
scoped to its own repo), which is why the notifications Bao cutover failed with
"Repository not found". bao-secrets is pure curl/jq logic — role_id/secret_id
come from the caller's Actions secrets — so nothing sensitive is exposed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -1,3 +1,25 @@
 # gitea-actions
 
-Shared Gitea Actions composite actions (public: logic only, no secrets). bao-secrets = OpenBao AppRole fetch. spikersoft-issues#545
+Shared **public** Gitea Actions composite actions for the SpikerSoft org.
+
+Public on purpose: these contain only workflow *logic* (shell/curl/jq), never
+secrets. Runners can clone this repo anonymously, which is why shared actions
+live here instead of the private `spikersoft-infrastructure` repo (a private
+repo can't be cloned by a job token scoped to a different repo).
+
+## Actions
+
+- **bao-secrets** — AppRole-login to OpenBao and export requested KV v2 secrets
+  into the job env (masked). Phase 1, spikersoft-issues#545.
+
+  ```yaml
+  - uses: https://git.spikersoft.com/spikerj/gitea-actions/bao-secrets@master
+    with:
+      role-id:   ${{ secrets.BAO_ROLE_ID }}
+      secret-id: ${{ secrets.BAO_SECRET_ID }}
+      secrets: |
+        DOCKER_PASSWORD  secret/ci/shared/registry  password
+  ```
+
+Reference by full URL (`https://git.spikersoft.com/...`) so Gitea resolves it
+from this instance instead of github.com.