Fix screenshot comments sharing images and never being deleted - #1

Two independent defects in how the screenshot comments are produced and
cleaned up. Both surfaced in gitx/gitx#658, #661 and #666.

1. Each PR's images overwrite every other PR's (b1a7454)

Give each uploaded diff image a name of its own, so a comment keeps showing
the run that wrote it.

The upload target is one flat directory shared by every project using this
script, and the images were sent under their bare basename, so
https://www.mxtracks.info/github/uploads/00-commit-context-menu.png is a
single global slot. The last run to finish replaces the images that earlier
comments already link to. Two pull requests compared at once end up rendering
the same pictures - and since the directory is shared across repositories,
AndroidChart and GitX can clobber each other the same way.

• Scope the uploaded name to the repository, the pull request and the arch,
  the way the error-screenshot branch just below already renames its file.
• Restore the rename the upload depends on. curl -F "file=@..." sends the
  basename as the stored name, so the file has to be renamed on disk for the
  URL to change - which is presumably why the name was left bare when the
  mv was commented out.

$GITHUB_REPOSITORY is already used at lines 61, 92 and 160, $PR is
computed at line 47, and $emulatorApi comes from the calling workflow, so
nothing else in the script changes.

2. Old comments are never deleted (1f4c399)

Read the comments payload without letting the shell rewrite it, so the
cleanup can find the comments it is meant to remove.

This script runs under zsh, whose echo expands backslash escapes. Line 74
pipes the comments JSON through echo, so every \n inside a comment body
becomes a real newline in the middle of a JSON string and jq refuses the
payload:

jq: parse error: Invalid string: control characters from U+0000 through
U+001F must be escaped at line 101, column 1

The || echo "" then leaves an empty list, so the cleanup reports "nothing
to do" while the old comments accumulate on every run.

It only bites once a pull request carries a comment written by a person. The
comments this script posts use <br/> and contain no escapes, which is why
it worked until now.

• Pass the JSON to jq with printf '%s' instead of echo.
• Apply the same change to the DEBUG_INFO dump, which printed the same
  corrupted JSON.

Lines 76 and 82 keep their echo, since those act on jq's own output, which
is ids and true/false with no backslashes.

Test plan

• zsh -n screenShotCompare.sh passes.
• Running the upload loop with the script's surrounding variables gives
  gitx-gitx-pr658-arm64-00-commit-context-menu.png,
  gitx-gitx-pr666-arm64-... and
  AppDevNext-AndroidChart-pr658-29-... where all three previously produced
  00-commit-context-menu.png.
• Replaying the full deletion chain (lines 74-83) under zsh against the live
  comment payloads of Screen shot false test gitx/gitx#661 and #666: the patched version finds all 7
  stale bot comments on #661 and both on #666, and correctly leaves the human
  comments alone. The current version returns an empty list for both.
• I have not been able to exercise the upload itself, since it needs the
  mxtracks credentials. Worth one real run on two concurrent pull requests.