fix: re-read frontmatter before modal save to prevent overwriting concurrent task edits

[martin-forge]

Problem

A background update to a task file can be undone when an already-open task edit window saves an unrelated change. For example, changing an estimate can restore an old scheduled date after a recurring task was completed elsewhere.

Solution

Read the latest frontmatter inside Obsidian's save callback and apply only the requested fields. Preserve background edits, completed occurrences and fields another writer removed. This changes the task update service only; it does not depend on the Calendar or security proposals.

Testing

• 110 focused tests pass, including a stale-edit regression and existing task/recurrence tests.
• npm run lint and npm run build:test pass.
• Reloaded the standalone build in a disposable Obsidian vault; no captured errors.
• GitHub CI: full test suite and build passed.