fix(ci): serve a loopback-base_url build to browser-based gates

[forkwright]

Finding

ci/github-workflow.yml.tmpl already builds a public-local/ copy with --base-url http://127.0.0.1:8080 and serves that to pa11y/playwright — but ci/kanon-ci.toml.tmpl (which its own header claims "mirrors ci/github-workflow.yml.tmpl stage-for-stage") and bin/typikon-check (the local pre-push gate) both still build once and serve public/, the production build, to the browser-based gates.

Evidence

• ci/kanon-ci.toml.tmpl [stages.serve-public] still cd public — the production build — before pa11y/playwright run.
• bin/typikon-check stages 6-7 (pre-fix numbering) still cd public && python3 -m http.server 8080 for both pa11y and playwright.
• Confirmed empirically: building examples/sample-blog and rebuilding with --base-url http://127.0.0.1:8080 --output-dir public-local shows every get_url()-derived reference (canonical, og:url, JSON-LD, sitemap) correctly rebases to loopback in the second build but not the first — proving the two builds genuinely differ, and confirming which one browser gates need to see.

Correction

Mirrors the already-fixed GH Actions recipe into both remaining implementations: a zola-build-local stage builds public-local/ (--force, since typikon-check reruns against the same checkout on every local invocation and zola refuses to reuse an existing --output-dir unprompted — caught by the new test below), and pa11y/playwright now serve that copy. public/ is untouched for csp-enforce, lychee, and deploy. public-local/ added to .gitignore alongside public/. docs/AGENTIC.md's stage list updated to match.

Test

New ci/local-base-gate-check.sh, wired into ci/run-fixtures.sh (which CI already runs via .github/workflows/gate-attestation.yml and .kanon-ci.toml): asserts public-local/ exists and its *.html files carry zero references to the site's configured production host. Scoped to .html (same rationale as ci/csp-enforce.sh) — atom.xml's <author><uri> is a legitimate hand-authored absolute URL by Atom convention and isn't fetched by a browser gate, so checking it would flag correct output as a regression.

Verified locally: on the pre-fix bin/typikon-check, public-local/ is never created and the new check fails with exit 1 (does not exist — the loopback-base_url build never ran); against this branch it passes for both examples/sample-blog and examples/sample-shop, and reruns idempotently against a leftover public-local/ from a prior run.

Refs #29