Workbook import phase 1: manifest-driven batch command

Implements the 18.09. strategy's phase 1 on top of the existing single
importer (which stays the one write path):

- new vitec:import-product-workbooks: runs vitec:import-product-workbook
  per manifest entry (in-process sub-command), aggregates a per-file
  report (writes/unchanged/na, URL-mismatch and error flags, details on
  -v or failure) plus a summary; without --apply every run is a dry
  run; --only filters
- migrations/products_xlsx/ now holds working copies of all 100 agency
  workbooks in their category folders (git-ignored) plus the TRACKED
  manifest.json: workbook path -> product slug, family entries mark the
  K-level landing pages (never imported), product null = unresolved.
  Generated from the 18.09. analysis + sitemap column K:
  68 product / 17 family / 15 open
- --init-manifest resolves open entries against the full product table
  (hidden records included): URL segment/filename vs slug, normalized
  title, unique slug prefix; only unambiguous hits are written back
- single command compare is now tolerant of the RTE storage transform
  (whitespace collapse + <p>-wrap simulation), fixing the known
  every-run-WRITE false positive on re-imports

Post-dry-run hardening, applied the same day (69 products, 785 field
writes across two runs, zero errors, verified against the live JSON):

- PROTECTED_FIELDS title/urltitle in the single command: the stored
  module mapping would have overwritten six canonical sitemap-V2
  product titles
- the stored mapping predated the 18.09. defaults and silently dropped
  `highlights` on every product; the single command now merges
  DEFAULT_MAPPING under the stored mapping (defaults as base, stored
  as per-field override)
- known limitation for phase 2: the g: composed fields report WRITE on
  every run (RTE transform alters ul/strong markup beyond the p-wrap
  normalisation) - the change watch must compare against stored
  imported values, not the database

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -122,3 +122,9 @@ CLAUDE.local.md
 
 # local developer scratch notes
 .dev-notes.md
+
+# Agency product workbooks - working copies synced from Nextcloud
+# (Material/Products). The manifest IS tracked: it documents which
+# workbook feeds which product record.
+migrations/products_xlsx/*
+!migrations/products_xlsx/manifest.json