feat(post-form): U1 — shared create/edit form + published field + owner_username

One /post form now drives both create and edit (KTD1, Alt B): cache-on-save
toggles add-page-by-form's overwrite_mode per submit — 'edit' when the new hidden
edit_path field is filled (write back in place, stable URL), 'false' when empty
(fresh dated folder via slug_field). add-page-by-form stays stock (no fork);
the toggle lives in cache-on-save, which reads edit_path and mutates the /post
page header's pageconfig before add-page-by-form's onFormProcessed runs.

- post-form.md: overwrite_mode:false (create-safe fallback), hidden edit_path,
  authoritative published toggle (default on), removed the static
  pagefrontmatter.published so the field wins on every submit (KTD3).
- cache-on-save.php: editPathFromForm + setOverwriteMode helpers.
- site.yaml: owner_username (single source of truth for the KTD8 owner gate).

Verified against the 2.0.4 container: create writes a fresh dated folder with
published:true/false as a real boolean (V1, V2); edit writes in place with the
slug/URL unchanged; save-as-draft-on-create works.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H1FrCYNq6RXdGYbn5PFrhM

config/site.yaml

@@ -7,3 +7,8 @@ metadata:
 description: 'A travel blog by Mischa'
 active_trip: /trips/us-canada-mex-2024
 travelling: false
+# Single source of truth for the site owner's account username. Backs both the
+# front-end edit/delete UI gate and the server-side scope guards (KTD8): only
+# this user (not merely any authenticated/super-admin account) may edit, delete,
+# or see drafts on the active trip's feed.
+owner_username: mischa