docs(voice): land Phase 1 voice profile derived from git-commit corpus

Phase 1 of the writing voice profile TODO (filed 7a861ed). The work
covers corpus assembly, statistics, and a cross-check against the 41
SKILL.md patterns. Email, PR, Slack, and long-form sources deferred to
Phase 2.

Corpus: 5355 commits, 1895 with non-trivial bodies, 128608 words across
33 repos. Strong findings:
- Pattern 17 (no emojis), Pattern 7 (AI vocabulary), Pattern 22
  (filler), Pattern 32 (first-person), Pattern 34 (contractions), and
  Pattern 38 (terse cut) are all confirmed by direct corpus
  measurement.
- Pattern 13 (em-dash zero-tolerance) and Pattern 33 (semicolons to
  period) contradict the corpus. Craig USES em-dashes at 3.49 per 1000
  words and semicolons at 3.16 per 1000 words, rates comparable to
  AI-generated prose. The rules are self-discipline, not
  habit-reflection. SKILL.md should say so honestly.
- Pattern 7 watch-word "comprehensive" appears 42 times in the corpus
  while every other watch-word clocks zero or one. "comprehensive" is
  genuine Craig vocabulary. The rule should pull it from the watch-list
  or flag only when it co-occurs with other AI tells.

New patterns the corpus suggests adding: single-sentence-paragraph
cadence (41.1% of paragraphs are exactly one sentence), parenthetical
density (23 opening parens per 1000), declarative-default register
(0.33 question marks per 1000).

Six concrete SKILL.md edits proposed in the doc, none applied. The
deltas await Craig's call.

Phase 2 sources are documented in the doc body.

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