Add pr-visual-writeup skill to .agents/skills - #1354

Verify each finding against the current code and only fix it if needed.

Inline comments:
In @.agents/skills/pr-visual-writeup/references/gist-upload.md:
- Around line 48-50: The fenced code block containing the URL template should
include a language tag to satisfy markdownlint MD040; update the block that
currently contains
"https://gist.githubusercontent.com/<user>/<gist-id>/raw/<filename>" so the
opening fence is ```text (i.e., add the "text" language identifier) to the
existing fenced code block.

In @.agents/skills/pr-visual-writeup/scripts/upload_gist.sh:
- Around line 42-56: The script currently copies files into "$WORK/gist/" and
lets later cp overwrite files with the same basename; update the loop that
iterates over input dirs (the for d in "$@"; do / for f in "$d"/*; do block) to
detect basename collisions before copying: compute b=$(basename "$f"), check a
tracking map (e.g. an associative array or temporary file keyed by "$b") to see
if b was already seen, and if so print a clear error or warning to stderr
(including both source paths) and exit non‑zero (or continue if you choose to
only warn); otherwise record the basename -> source and proceed to copy and
increment count; ensure messages use >&2 and preserve the existing staged count
behavior.
- Around line 64-66: The current git invocation in upload_gist.sh embeds the PAT
in the command argv via -c credential.helper="!f() { echo username=$USER; echo
password=$TOKEN; }; f", exposing the token to other same-UID processes; change
the flow to avoid passing $TOKEN on the command line by using GIT_ASKPASS (or an
askpass helper read from stdin) instead of the inline credential.helper: create
a small askpass helper that prints the password to stdout when invoked and set
GIT_ASKPASS to its path (or export a process-safe askpass via a here-doc) before
invoking git push, and remove the inline -c credential.helper snippet so the
token is never present in the git argv.

In @.agents/skills/pr-visual-writeup/SKILL.md:
- Around line 129-137: Markdownlint flagged MD040 for the directory tree fenced
code block in .agents/skills/pr-visual-writeup/SKILL.md (the block starting with
"/tmp/pr-<N>-visuals/"); fix it by adding a language identifier to the opening
fence (e.g., change the opening "```" to "```text") so the block becomes a
labeled code block, ensure the closing fence remains present, and apply the same
fix to any other unlabeled fenced blocks in that file.

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Nitpick comments:
In @.agents/skills/pr-visual-writeup/scripts/detect_dev_server.sh:
- Around line 26-29: The script currently prints "no listening node servers in
$MIN-$MAX" to stderr but returns exit code 0, which is ambiguous for callers;
change the exit behavior so that when the variable "ports" is empty (the if [ -z
"$ports" ] branch) the script exits with a non-zero status (e.g., exit 1) so
consumers can detect "no servers found"; update the echo/exit in that branch
accordingly and ensure any tests or callers expecting success are adjusted to
treat the non-zero exit as "no servers".

In @.agents/skills/pr-visual-writeup/scripts/upload_gist.sh:
- Around line 69-73: The script currently writes the GIST_ID to
"$OLDPWD/gist-id.txt", which is fragile; capture the current working directory
into a stable variable (e.g. ORIGINAL_PWD="${PWD}") before the internal cd
"$WORK/gist" and then write the id to "$ORIGINAL_PWD/gist-id.txt" instead of
"$OLDPWD"; update the echo that outputs the raw URLs if it should reference the
same directory variable or leave it relative to the script's working directory,
but replace any use of "$OLDPWD" with the saved ORIGINAL_PWD variable and ensure
GIST_ID is written to ./gist-id.txt as promised.