feat(render): foldable Pearl help header, dropping the # comments

The preamble carried two raw "# ..." affordance comment lines on every Linear file. Those move into a folded "* Pearl" heading, gated by a new pearl-help-header defcustom (default t). It collapses on open via a :VISIBILITY: folded property and carries the edit-then-save reminder, a note that the section is never sent to Linear, the key cues (derived from pearl-keymap-prefix, falling back to M-x when no prefix is bound), the priority-cookie legend, and how to hide it. Set the defcustom to nil for a minimal preamble of just the machine keywords. The heading also doubles as the "you're in Pearl" marker at the top of the buffer.

The heading has no LINEAR-ID, so save and merge skip it for free: they walk only LINEAR-ID-bearing subtrees. A forward declaration of pearl-keymap-prefix keeps the byte-compiler happy, since its defcustom lives later in the file than the renderer.

This is the human-content half of the preamble work. The machine #+ keyword lines (title, STARTUP, TODO, LINEAR-SOURCE, the provenance fields) are still visible. Hiding those is the remaining part of the task.

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