docs: READMEs describe current behaviour, not history

Removes the decision logs, the dated rows, the strikethrough retirements and
the setup progress checkboxes from all four READMEs. A decision log records how
the system came to be; a reader needs to know how it is. The two had drifted
far enough apart that the docs asserted things the shipped config contradicted.

Corrected, all of which were stale rather than merely verbose:

- the architecture section described the source-repo workflow as self-contained
  and called the reusable workflows here "not actively called; may be stale".
  They are the pipelines; the source-repo workflow is a thin caller.
- `workflow_call` was recorded as abandoned, and the instance as having no orgs.
  Both pipelines run through cross-repo `workflow_call` under an org.
- tech-blogs was described as not yet initialised. It has published.
- ses and tech invocation sections claimed a self-contained workflow too.

Kept, rewritten as behaviour rather than as dated decisions, because they are
live operational constraints and not history:

- the callee workflow cache, why a `publish-*.yml` edit appears not to take,
  and the flush command with its `--no-deps` and `--pull` caveats
- why this repo has to stay public
- the `URL_TO_GITEA` / `PUBLIC_URL_TO_GITEA` split and what validates it
- host network mode, and that `kaniko-act` cannot be rebuilt by build-image.yml

Also documents what the renderer, its failure modes and its publish scope
actually are, which the decision logs had been standing in for, and adds the
architecture note that renderer code is cloned fresh per run while workflow
YAML is cached — the distinction that governs whether a change takes effect.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pipelines/songs/SCHEMA.md

@@ -268,4 +268,4 @@ Files without a `wiki.publish: true` frontmatter field are silently skipped —
 
 Validation and rendering both run before any wiki write. A failure at either stage aborts the run with a non-zero exit and leaves the wiki untouched, including when only some of the run's pages are broken.
 
-Frontmatter keys the schema does not define are ignored, so a leftover `wiki.siblings` block from the retired macro system is inert rather than an error. Delete them; they do nothing.
+Frontmatter keys this schema does not define are ignored rather than rejected.