Add archify skill (v2.14), pi-mermaid ext (v0.3.0), project-diagramming-mermaid + project-diagramming-archify skills

- skills/archify: vendored tt-a1i/archify 2.14.0 (MIT), pure Node 18+, zero deps
- extensions/pi-mermaid: renders mermaid blocks as ASCII in TUI; deps beautiful-mermaid + mermaid
- skills/project-diagramming-*: orchestration skills, per-project docs/ convention

skills/archify/renderers/shared/text-fit.mjs

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+// Single-line node text fitting, shared by every renderer.
+//
+// Node text (`label`, `sublabel`, `tag`) renders as one <text> element with
+// text-anchor="middle" and is never wrapped. Left unmeasured, an over-long
+// value silently spills across its neighbours while validation still reports
+// a clean receipt — the failure mode this module exists to close.
+//
+// Two halves, always used together:
+//   - fittedNodeFontSize shrinks the text toward a legible minimum at render
+//     time, so ordinary overruns simply get smaller instead of overlapping.
+//   - minimumNodeTextWidth reports the width the text still needs once it has
+//     shrunk as far as it may, so validation can reject what shrinking cannot
+//     save.
+//
+// The geometry constants below are shared; the per-field `preferred` and
+// `minimum` font sizes are not, because renderers set node text at different
+// sizes (architecture sublabels are 9px, the rest are 7px).
+
+import { textUnits } from './utils.mjs';
+
+// widthFactor: px of advance width per text unit, per px of font size.
+// horizontalPadding: total px reserved inside the box so text never touches
+// the border.
+export const nodeTextFit = {
+  widthFactor: 0.6,
+  horizontalPadding: 8,
+};
+
+// Largest font size at or below `preferred` that fits `text` inside `width`,
+// floored at `minimum` — below that the text is no longer legible and the
+// caller should be reporting a problem instead.
+export function fittedNodeFontSize(text, width, preferred, minimum) {
+  const units = Math.max(1, textUnits(text));
+  const available = Math.max(1, width - nodeTextFit.horizontalPadding);
+  const fitted = Math.min(preferred, available / (units * nodeTextFit.widthFactor));
+  return Math.max(minimum, Math.floor(fitted * 10) / 10);
+}
+
+// Width `text` occupies at its legible minimum. Compare against
+// `width - nodeTextFit.horizontalPadding` to decide whether shrink-to-fit can
+// rescue it.
+export function minimumNodeTextWidth(text, minimum) {
+  return textUnits(text) * minimum * nodeTextFit.widthFactor;
+}
+
+// Available text width inside a box of `width`.
+export function availableNodeTextWidth(width) {
+  return width - nodeTextFit.horizontalPadding;
+}