docs: P4 skill sweep - dtfmt, mod signedness, multi-line fn, 3-form cond

[danieljohnmorris]

Summary

Four doc gaps from the P4 backlog, all in skill files (no source changes):

• feat: add min and max builtins #23 SKILL.md: text builtins summary now names dtparse dtfmt - multiple personas missed these because "date parsing" gave no signal to load ilo-builtins-text
• feat: add flr and cel builtins for floor/ceil #24 ilo-builtins-math.md: one-line note that mod is C-style signed (mod -1 7 = -1), with the +7 mod 7 floor-mod pattern - every TZ/weekday persona hit this silently
• feat: migrate Err-wrap sigil from ! to ^ #25 ilo-language.md: explicit note + example that multi-line fn bodies require trailing ; on the name>type; header line - caused confusing parse errors
• feat: add !x as logical NOT expression #26 ilo-language.md: side-by-side table of all three conditional forms (?h cond a b / braceless guard / cond{body}) with the common mistake called out - all ten date personas tripped on this

What's in the diff

• docs: add dtfmt/dtparse to SKILL.md text builtins summary
• docs: document mod signedness gotcha in ilo-builtins-math
• docs: document multi-line fn body header semicolon requirement (includes the 3-form cond table - both Guards section changes)

Test plan

• ilo skill get ilo-builtins-math contains the mod signedness note
• ilo skill get ilo-language shows the multi-line fn header example and the 3-form cond table
• ilo skill get ilo (SKILL.md bootstrap) shows dtparse dtfmt in the text builtins line
• No source files changed - pure skill doc update

Follow-ups

P1 #9 (mod floor-mod alias fmod) and P1 #6 (parse error improvement for wrong cond form) are tracked separately in pending.md.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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