…tter (PRD-7216) (#2295)

* fix(open-knowledge): accept Obsidian-style empty tags/aliases frontmatter (PRD-7216)

Obsidian writes an empty `tags:` / `aliases:` list as a one-element block
sequence whose only item is blank (`tags:\n- `), and a bare `tags:` as a
null scalar. yaml@2 parses both to `null`, which the frontmatter read
schema excluded (null is the RFC 7396 merge-patch delete sentinel). Because
parseFmRegion validates the whole map, one empty list rejected the entire
block: the Properties panel showed nothing and refused every edit, even to
the file's other valid keys. ~92% of a real Obsidian vault is affected.

Coerce these null shapes to empty values at the single read-path boundary
(z.preprocess on FrontmatterMapSchema): null seq element -> dropped
([null] -> []), null mapping value -> '' (empty text). Both structured
readers (parseFmRegion for the panel; parseFrontmatterYaml for disk, the
agent-write gate, and the tag indexer) inherit it, so they can't drift.
The coercion rewrites only the JS view; the yaml CST the write path mutates
is untouched, so files aren't rewritten on disk until the user edits a
property. null stays out of the per-value write/patch schema, so it still
means delete in RFC 7396 patches.

Whole-block reflow on first edit (PRD-7217) is separate and out of scope.

* review: drop session-stat/PRD markers from source comments; pin Obsidian-null at the agent-write HTTP gate

Addresses claude[bot] review on #2295:
- Remove the `~92%` session statistic from the `coerceNullFrontmatter`
  JSDoc and the region test comment (it lives in the commit message; in
  source it rots without a reproducible query — OK comment discipline).
- Drop the new `PRD-7217` marker from a test comment; keep the substance
  ("a separate, pre-existing concern").
- Add an integration test: a `replace` agent-write of Obsidian's
  `tags:\n- ` / `aliases:\n- ` shape returns 200 (not 400) and reads back
  coerced, pinning the HTTP boundary so a future early-guard before the
  schema call can't silently regress it.

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GitOrigin-RevId: 372b7b9e97091b67debc96a574bed54cec02d923