Add pr-description skill for short GitHub write-ups (#19891)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro

.github/skills/pr-description/SKILL.md

@@ -0,0 +1,98 @@
+---
+name: pr-description
+description: Use when drafting, proposing, creating, or editing prose for a dotnet/fsharp GitHub PR or issue — body, title, comment, review summary, edits — including bare asks like "open a PR", "ship this", "write up what I did", "summarise the change", "reply on the PR", "edit the issue body", "gh pr create", "gh pr comment", "gh pr edit --body", "gh issue comment", "gh pr review --body". Primary use case is PR descriptions; same rules apply to PR/issue comments and review summaries. Not for labels, reviewers, merging, or code-review findings (just the prose write-up of them).
+---
+
+# Authoring GitHub Prose for dotnet/fsharp
+
+Reviewers can already see the Files tab, the commit log, and the issue thread. Say what behavior changed and why, in as few words as possible.
+
+## Rules
+
+Rules 1, 2, 4, 5 are defaults; if the user insists, push back once then comply. Rule 3 is non-negotiable — `-b "..."` ships broken markdown (see PR #19866).
+
+1. **No change inventory.** No file/module/method/test lists. No `## Changes`/`## Implementation` section. Mention an identifier only when it *is* the user-visible behavior. Whatever the reader already has (Files tab for PRs, commit log for follow-up comments, issue history for issue edits) — don't re-list it.
+2. **No LLM slop, no justification scaffolding.** No emoji headers, no "TL;DR" above a 3-line body, no Motivation/Background/Approach/Testing sections, no re-stating the title or the comment you're replying to. No "matching the X norm", no "preventing the Y failure (PR #ZZZZ)", no stats, no links to past PRs as proof. The diff is the proof.
+3. **Body via `--body-file`, built without shell expansion.** Write the file with your file-creation/edit tool (it writes bytes verbatim — no `$`/backtick evaluation, no delimiter collisions, OS-agnostic). Never `-b "..."` / `--body "..."` — backticks and `$` get shell-evaluated and the render breaks. If you build the file in a shell, use a pwsh verbatim here-string `@'...'@` (cross-platform; single-quoted is mandatory). Applies to `gh pr create/edit/comment/review`, `gh issue create/edit/comment`.
+4. **`Fixes #N` to close issues.** Use only when the PR actually closes #N (auto-closes on merge). It is the highest-value line in most PR bodies — never omit it when valid. No "Related to" / speculative links. Preserve existing trailers (`Co-authored-by:`, `Signed-off-by:`, `Reverts #N`); don't invent them.
+5. **Title:** imperative, ≤72 chars, no trailing period, no `fix:`/`feat:` prefix. Name the behavior, not the file. A specific title lets the body shrink to `Fixes #N` + one sentence.
+
+## PR-body shapes (pick the smallest that carries the signal)
+
+**One-liner** for bumps / mechanical fixes:
+~~~
+Update .NET SDK from 10.0.202 to 10.0.204.
+~~~
+
+**Title carries the construct + issue link** — preferred when the title is specific:
+~~~
+Fixes #18009
+
+Wrong colorization when a qualified type name with generic parameters
+is used in a static member access expression.
+~~~
+
+**Issue link + 1-sentence why** — the most common non-trivial shape:
+~~~
+Fixes #19751
+
+`--refout` MVIDs were unstable because hashing relied on per-process
+string randomization. Switched to a deterministic hash.
+~~~
+
+**Before/After code block** — when prose loses information; ≤15 lines, language tag:
+~~~markdown
+Fixes #15803
+
+```fsharp
+let f (x: int) = x <- 1
+// before: FS0027 suggested `let mutable x = expression` (illegal for parameters)
+// after:  FS0027 suggests `let mutable x = x` shadow or `byref<_>`
+```
+~~~
+
+**Behavior-changes bullet list** — for genuinely multi-behavior PRs. 3–5 bullets naming *behaviors*, never files/modules; if bullets map 1:1 to files, collapse to prose or split the PR:
+~~~
+Fixes #19710
+Fixes #19720
+
+- `match x with null` now preserves type aliases.
+- FS0027 on parameters suggests `let mutable x = x` shadow / `byref<_>`.
+- `let _ = &expr` compiles like `let x = &expr`.
+~~~
+
+Prefer inline backticks for short identifiers over a fenced block. Fenced blocks are the exception — use only when prose loses information.
+
+**Comments / review summaries:** same rules, no title. Usually 1–3 sentences. Quote-reply only the line you're responding to; don't restate it.
+
+## Workflow
+
+Show the title + body (or comment text) in chat first. **Do not run `gh` until the user approves.** Then:
+
+1. **Write the body to a file** with your file-creation tool (bytes verbatim, OS-agnostic, avoids every shell-quoting trap). Or in pwsh:
+
+   ```powershell
+   @'
+   Fix false-positive FS3261 when nullness narrowing leaks across iterations
+   of seq/list/array comprehensions.
+
+   Fixes #19644
+   '@ | Set-Content -NoNewline pr-body.md
+   ```
+
+2. **Post** with exactly one:
+
+   ```
+   gh pr create  --title "<imperative title>" --body-file pr-body.md
+   gh pr edit    <N> --body-file pr-body.md     # REPLACES body — read first if extending
+   gh pr comment <N> --body-file pr-body.md
+   gh pr review  <N> --body-file pr-body.md --comment   # or --approve / --request-changes
+   gh issue create  --title "<t>" --body-file pr-body.md
+   gh issue edit    <N> --body-file pr-body.md
+   gh issue comment <N> --body-file pr-body.md
+   ```
+
+   To extend without losing the existing body:
+   `gh pr view <N> --json body -q .body > pr-body.md` → edit → `gh pr edit <N> --body-file pr-body.md`.
+
+3. **Verify** the live render — fetch the body and compare to the file with your view/diff tool. Fail loudly if `gh` fetch errors (a piped failure prints an "all-deleted" diff that misreads as "GitHub mangled it").