docs(voice): split SKILL.md and voice-profile.org into paired source-of-truth, Pattern 13 worked example

This is option C from the structural-split proposal. SKILL.md becomes
the thin rule-set. voice-profile.org becomes the canonical home for
rationale, basis, examples, and per-pattern history. Pattern 13 is
migrated as the worked example to confirm the form. The remaining 40
patterns are scheduled for a backfill pass.

SKILL.md gains a Source of Truth section near the top stating the
pairing rule. Every change to a pattern must land in both files. A
SKILL.md edit without a profile update is incomplete. A profile
update without a SKILL.md edit is fine.

Pattern 13 in SKILL.md collapses to a single Rule line plus mode tags
plus a pointer to voice/references/voice-profile.org §13. The Problem
paragraph, the Mode-dependent-strength paragraph, the Note-on-basis
paragraph, and the Before/After examples all move to the profile.

voice-profile.org gains a How this combines with SKILL.md preamble
that names the pairing rule explicitly. Pattern §13 lands with all
the migrated content plus a History section recording the prior
commits (original SKILL.md entry, the 2026-05-26 prose-mode addition
in 4fac2a0, the 2026-05-29 basis note in c3cf9a5, and this
migration).

If Craig confirms the shape, the next pass backfills patterns 1
through 12 and 14 through 41 in the same form.

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