docs(references): port style guides, 24 diagram guides, and 9 templates from opencode

All content ported from borealBytes/opencode under Apache-2.0 license with
attribution headers prepended to each file.

- references/markdown_style_guide.md (~733 lines): full markdown formatting,
  citation, collapsible sections, emoji, Mermaid integration, and template
  selection guide
- references/mermaid_style_guide.md (~458 lines): full Mermaid standards —
  emoji set, classDef color palette, accessibility (accTitle/accDescr),
  theme neutrality (no %%{init}), and diagram type selection table
- references/diagrams/ (24 files): per-type exemplars, tips, and templates
  for all Mermaid diagram types
- templates/ (9 files): PR, issue, kanban, ADR, presentation, how-to,
  status report, research paper, project docs

Source: https://github.com/borealBytes/opencode

scientific-skills/markdown-mermaid-writing/references/diagrams/xy_chart.md

@@ -0,0 +1,53 @@
+<!-- Source: https://github.com/borealBytes/opencode | License: Apache-2.0 | Author: Clayton Young / Superior Byte Works, LLC (Boreal Bytes) -->
+
+# XY Chart
+
+> **Back to [Style Guide](../mermaid_style_guide.md)** — Read the style guide first for emoji, color, and accessibility rules.
+
+**Syntax keyword:** `xychart-beta`
+**Best for:** Numeric data visualization, trends over time, bar/line comparisons, metric dashboards
+**When NOT to use:** Proportional breakdowns (use [Pie](pie.md)), qualitative comparisons (use [Quadrant](quadrant.md))
+
+> ⚠️ **Accessibility:** XY charts do **not** support `accTitle`/`accDescr`. Always place a descriptive _italic_ Markdown paragraph directly above the code block.
+
+---
+
+## Exemplar Diagram
+
+_XY chart comparing monthly revenue growth (bars) versus customer acquisition cost (line) over six months, showing improving unit economics as revenue rises while CAC steadily decreases:_
+
+```mermaid
+xychart-beta
+    title "📈 Revenue vs Customer Acquisition Cost"
+    x-axis [Jan, Feb, Mar, Apr, May, Jun]
+    y-axis "Thousands ($)" 0 --> 120
+    bar [20, 35, 48, 62, 78, 95]
+    line [50, 48, 45, 40, 35, 30]
+```
+
+---
+
+## Tips
+
+- Combine `bar` and `line` to show different metrics on the same chart
+- Use **emoji in the title** for visual flair: `"📈 Revenue Growth"`
+- Use quoted `title` and axis labels
+- Define axis range with `min --> max`
+- Keep data points to **6–12** for readability
+- Multiple `bar` or `line` entries create grouped series
+- **Always** pair with a detailed Markdown text description above for screen readers
+
+---
+
+## Template
+
+_Description of what the X axis, Y axis, bars, and lines represent and the key insight:_
+
+```mermaid
+xychart-beta
+    title "📊 Your Chart Title"
+    x-axis [Label1, Label2, Label3, Label4]
+    y-axis "Unit" 0 --> 100
+    bar [25, 50, 75, 60]
+    line [30, 45, 70, 55]
+```