Frontmatter writer injects empty title/created onto records that lack them (events)

[williamthorsen]

Description

writeFrontmatter (in @codeassembly/kb) renders title, created, and updated unconditionally, in a fixed order. The Frontmatter model treats them as first-class string fields, and parse-note.ts defaults each to '' when the source omits it. This is assertion-shaped: assertions carry title/created/updated, but event records do not (they use captured-at, have no title, and — until #809 — no updated).

So writing an event back through writeFrontmatter injects title: '' and created: '' into it. Validation then rejects the record, because created is a date field (DATE_FIELDS) and '' is not a valid date:

created: expected YYYY-MM-DD or YYYY-MM-DDTHH:MM:SSZ, got ""

writeBackNote validates before the atomic write, so the write is refused and the note is left untouched — which means every kb-edit operation on an event fails outright. --add-addressed-by (#785) is the first operation pointed at events, so it is the first to surface that the write path cannot round-trip a non-assertion record.

This is independent of the schema declarations in #809: even with updated/addressed-by declared on the event type, the writer still injects an empty title/created, and the date rule still rejects the empty created.

Context

• Files: packages/kb/src/frontmatter/write-frontmatter.ts (unconditional emit), packages/kb/src/frontmatter/parse-note.ts (defaults to ''), packages/kb/src/types.ts and frontmatter-schema.ts (first-class fields), packages/kb/src/rules/frontmatter-rule.ts (DATE_FIELDS).
• The fix is for the writer to emit title/created/updated only when the record actually carries them, rather than always — so it round-trips events (and any future non-assertion record) without injecting empty values. The deeper modeling smell — assertion-specific fields baked into a shared Frontmatter type — can be resolved here or noted for later.
• Surfaced while marking the five Agents include non-finding findings in review #784 events addressed-by; blocks that marking.
• Related: event schema doesn't declare updated/addressed-by, so kb-edit writes undeclared fields #809 (event schema declarations — necessary but not sufficient), Add a kb-edit operation to append addressed-by references to records #785 (the operation that exposed it), Agents include non-finding findings in review #784 (event-marking blocked on this).

Acceptance criteria

Must have

• writeFrontmatter round-trips an event record without injecting title, created, or updated fields the record does not carry.
• A kb-edit operation (e.g. --add-addressed-by) on an event produces a record that passes schema validation, including the date-format rule (no empty created).
• Records that legitimately carry title/created/updated (assertions) still round-trip unchanged.
• New and modified behavior is covered by tests, including round-tripping an event through a write operation.

[williamthorsen]

Subsumed by the per-type record architecture epic (#848). The fix lands as a side effect of modeling events and assertions as distinct record types: events get their own write path (#849) that never emits assertion fields, and update-events (#850) — which closes this — exercises the round-trip. No standalone writer patch is planned.