feat: refactor the Skills detail pane into collapsible sections (#2263)

The pane was one scrolling column: SKILL.md sat below the manifest behind a
"View SKILL.md" button, the frontmatter pushed it further out of view, and
nothing could be put away.

It is now a flex column that does not scroll as a whole. Conformance,
Resources, Frontmatter and Skill Resource are four collapsible sections of one
`disclosure` accordion, and the file viewer claims the remaining height and
scrolls internally.

The viewer's flex-basis is 0, not auto, and that is load-bearing: `auto` makes
an item's basis its content height, and a rendered document is ~1,795px for the
data-analysis fixture. That basis joined the sum the container distributes,
over-constraining the column so the shrink factors crushed the siblings —
Conformance to a 2px panel, Resources to zero — with their contents spilling
over the headers below. Collapsing the viewer removed the giant basis, so the
symptom appeared on collapse-then-reopen. Open sections also carry a `mih`
floor, since the disclosure CSS sets `min-height: 0` on an active item.

Frontmatter now follows the displayed file rather than the selected skill.
Both halves come from one `splitSkillFile` call, so the section and the viewer
cannot disagree, and a file with no frontmatter renders no section at all.

Also: SKILL.md loads on selection (a `resources/read` is not a load under
SEP-2640), the manifest highlights on hover with each URI a button that swaps
the viewed file, both whole-skill actions moved to the pane header, and the
Conformance badge goes yellow for a warnings-only entry instead of green.

Flat CSS properties moved out of call sites into theme variants (ThemeText
gains four; a new ThemeTable takes the manifest font size).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R4441JdNLZerCVM7mDDSfB
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/utils/splitSkillFile.ts

@@ -0,0 +1,55 @@
+/**
+ * Split a skill file into its YAML frontmatter and its body (#2263).
+ *
+ * A pure transform with no I/O and no subsystem of its own, so it lives in
+ * `utils/` rather than `lib/` — and in its own module rather than in
+ * `SkillsScreen.tsx`, because a component file that also exports a function
+ * defeats React Fast Refresh (`react-refresh/only-export-components`).
+ */
+
+export interface SkillFileParts {
+  /**
+   * The raw YAML between the fences, fences excluded — `undefined` when the
+   * file has no frontmatter at all. Raw rather than parsed: this app carries no
+   * YAML parser, and showing the bytes the server actually served is the more
+   * useful answer for a conformance tool anyway.
+   */
+  frontmatter?: string;
+  /** Everything after the closing fence, or the whole file when there is none. */
+  body: string;
+}
+
+/**
+ * Separate a leading YAML frontmatter fence from the rest of a skill file.
+ *
+ * The Skills screen renders the two halves in different places — the
+ * frontmatter in its own collapsible section, the body in the file viewer — and
+ * deriving both from **one** split is what stops them disagreeing: the section
+ * can never show one file's frontmatter while the viewer shows another's, and a
+ * file with no frontmatter cannot leave a stale section on screen.
+ *
+ * It also matters for rendering: the markdown renderer has no frontmatter
+ * support, so an un-split `---\nname: …\n---` is read as a setext heading and
+ * painted as a title above the document's real one.
+ *
+ * Two deliberate conservatisms, because this must never eat content:
+ *
+ *   - Only a fence at the very **start** of the file counts. A `---` anywhere
+ *     else is a horizontal rule and is left in the body.
+ *   - A file that opens with `---` but never closes the fence is **not**
+ *     frontmatter; the whole file is returned as the body rather than being
+ *     truncated to nothing.
+ */
+export function splitSkillFile(text: string): SkillFileParts {
+  if (!/^---[ \t]*\r?\n/.test(text)) return { body: text };
+  const rest = text.slice(text.indexOf("\n") + 1);
+  const close = rest.search(/^---[ \t]*\r?$/m);
+  if (close === -1) return { body: text };
+  const frontmatter = rest.slice(0, close).replace(/\r?\n$/, "");
+  const after = rest.slice(close);
+  const newline = after.indexOf("\n");
+  if (newline === -1) return { frontmatter, body: "" };
+  // Drop the blank line conventionally left between the fence and the body, so
+  // the document does not open with dead space.
+  return { frontmatter, body: after.slice(newline + 1).replace(/^\r?\n/, "") };
+}