docs: document per-branch preview envs and build-time CNAME

Bring docs/deployment.md in line with the updated workflow: per-branch
preview environments, branch-keyed concurrency, inactive-then-delete
cleanup, and the fact that CNAME is now part of the build output.

Author: danielnaab

docs/deployment.md

@@ -17,18 +17,23 @@ The site is hosted on GitHub Pages with a custom domain.
 
 ## Workflow
 
-`.github/workflows/deploy.yml` runs on every push:
+`.github/workflows/deploy.yml` runs on every push, on branch delete, and on manual `workflow_dispatch`:
 
 1. Installs Bun, runs the test suite, builds the site with `SITE_BASE_URL` set to `/` for `main` or `/preview/<branch>/` for other branches.
 2. Runs the axe-core a11y scan against the built output.
 3. Checks out `gh-pages`, syncs the build output into the right directory, commits, and pushes.
-4. Registers a GitHub Deployment against the `production` or `preview` environment so the URL appears in the PR "Deployments" panel.
+4. Registers a GitHub Deployment against the `production` environment (for main) or a per-branch `preview/<sanitized-branch>` environment, so every branch's URL appears in the PR "Deployments" panel and the repo's Deployments page can be used to navigate per-branch preview state.
 
-When a branch is deleted, a cleanup job removes `preview/<sanitized-branch>/` from `gh-pages`.
+Concurrency is keyed on the acted-on branch name, not `github.ref`. A `delete` event against `feat/foo` and a `push` to `main` run in different concurrency groups, so a branch cleanup cannot cancel a production deploy.
+
+When a branch is deleted, the `cleanup-preview` job runs and does three things:
+- Removes `preview/<sanitized-branch>/` from `gh-pages`.
+- Marks every deployment in the `preview/<sanitized-branch>` environment as `inactive`.
+- Deletes the per-branch environment via the REST API so it drops out of the Deployments sidebar.
 
 ## Custom domain
 
-`CNAME` at the source repo root contains `labs.flexion.us`. The workflow copies it to `gh-pages` if absent.
+`CNAME` at the source repo root contains `labs.flexion.us`. The build driver (`src/build/entry.tsx`) copies it into `dist/` on production builds only — preview subdirectories must not carry a CNAME.
 
 ## Base path