Brain Page format: ## compiled_truth / ## timeline headings conflict with in-content headings

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Problem

The canonical Page format uses visible ## compiled_truth / ## timeline headings as structural markers (skills/brain-setup/assets/BRAIN.md:94,98, written by skills/brain-page/bin/brain.mjs:146,150). This conflicts with heading levels inside the content itself:

## compiled_truth        <- H2 structural marker

# Some Decision          <- H1 content heading, renders larger than its own container

## Background            <- H2, same level as the sibling "## timeline" marker below

## timeline               <- H2 structural marker

Two concrete issues:

1. Any H1 inside compiled_truth visually outranks the ## compiled_truth marker that's supposed to contain it — the hierarchy is inverted.
2. Any ## sub-heading inside compiled_truth sits at the same nesting level as the sibling ## timeline marker, so the structure is ambiguous both to a human reader and to any parser walking headings by level rather than by exact string match.

Proposed fix

Replace compiled_truth's visible heading with an HTML comment marker, <!-- compiled_truth -->. Content between it and the next marker is then free to use any heading level (#, ##, ###) with no collision, since the marker itself renders invisibly.

timeline must keep a visible heading rather than becoming a comment. An HTML-comment marker there would make the section unidentifiable when the page is opened in a plain Markdown editor/previewer with no knowledge of the brain format — there'd be nothing rendered to tell a reader "this is the timeline," and the entries would appear to run together with the preceding prose with no visual break. So timeline is normalized to ## Timeline (capitalized) instead of the current lowercase ## timeline, but stays a real heading — preceded by an <!-- timeline --> comment marker so the parser locates the section by the exact, stable comment rather than by matching heading text/level (which content authors could otherwise duplicate or rename).

Canonical shape:

---
id: page-id
...
---

<!-- compiled_truth -->

# Page Title

## Some sub-section

...

<!-- timeline -->

## Timeline

- time: ...
  kind: ...
  summary: ...

Any format change needs backward-compatible parsing for existing brain data written under the current ## compiled_truth / ## timeline shape.

Scope of work

• skills/brain-page/bin/brain.mjs — update serialization + parsing (lib/brain.mjs:169) to write the new marker shape while still accepting the old one on read.
• skills/brain-setup/assets/BRAIN.md — update the documented canonical Page structure.
• skills/brain-page/SKILL.md — update prose describing ## compiled_truth / ## timeline (currently lines 74 and 83).
• skills/brain-page/test/brain.test.mjs — update fixtures/assertions pinned to the old headings; add coverage for parsing both old and new forms.
• Consider a migration helper in the brain CLI that rewrites existing pages' markers in place (format-only, no content or timeline changes) so old data isn't silently stuck on the legacy shape.

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PR #9