Check every page the family edits against the schema of its type #192

Story

As a family member who edits the family's pages by hand (Forgejo, Obsidian, stack memory write), I want every page checked against the schema of its type, so that a mistake is named with its path instead of silently read as an empty page.

Context

lib/stack/frontmatter.py holds the format's parser and a schema per type (SCHEMAS, and DECLARED_SCHEMAS for the person files from #191). Two gaps remain:

• Types and fields without a schema. todos (a state document, docs/design/brain/vault-format.md §7) has none, so it cannot be validated at all. The validator knows required, list and single-value fields, but not allowed values: relation_kind lists nine values in vault-format §5 and anything passes.

• Readers that bypass the validator. They parse frontmatter on their own and turn an error into an empty dict, so a broken page reads as a page without metadata:

  • stacklets/memory/lib.py _parse_frontmatter wraps the strict parser and returns {} on any exception (search).
  • stacklets/memory/bot/cli/diary.py:388 and stacklets/docs/bot/vault_entry.py:668 parse with PyYAML and return {} on a YAML error.
  • stacklets/memory/bot/cli/wiki.py:164 uses PyYAML to decide whether a page's frontmatter is broken.

  PyYAML accepts more than the format allows, so these readers and the validator can disagree about the same file.

The person files in #191 show the pattern: one reader, strict parse plus validate(), a broken file passed to a report callback with its path and left out, and nothing destructive done while one is broken.

Proposal

• Add a schema for todos, and allowed values for enum fields (relation_kind first), in lib/stack/frontmatter.py, with vault-format.md updated in the same change.
• Move the four readers above onto stack.frontmatter.parse and validate(). A page that fails is reported by path where the reader's operator looks (curator log, stack memory prompt, archivist log) and left out, never read as empty.
• Drop PyYAML from the frontmatter paths it then no longer serves.

Out of scope: new page types, the dream cycle, changing the format itself.

Definition of done

• A todos.md with a field outside its schema is reported by path by the curator, and its todos are left untouched.
• A person page with relation_kind: cousin fails validate() with a message naming the allowed values.
• A vault entry with frontmatter outside the format is named by path in the archivist log, instead of surfacing in search without metadata.
• Caller-side tests pin each of the three, against the format in vault-format.md, not against the implementation.
• No frontmatter is parsed with PyYAML outside lib/stack/frontmatter.py; the archivist's message catalogue (archivist.py:190) is not frontmatter and stays.
• docs/design/brain/vault-format.md lists every schema the validator enforces.