Style improvements

[hseg]

(based on #2, on the presumption that that is sufficiently uncontroversial that this'll save rebase time)

In implementing #2, I found multiple corners for improvement. Not all of this is necessary, of course.

• Basic shellcheck improvements
  Dead code elimination, quoting

• Finish shellcheck corrections

  • Pull out common request-crafting code
  • Handle unhappy paths so the happy path is straightline code
  • Test exit codes directly to avoid them being masked

• Use traps to clean up temp files, use mktemp
  This is less error-prone than trying to remember to do so on all exit paths. Use mktemp to avoid collisions and enable the sysadmins to configure where to put temp files.

• Use symbolic escapes instead of codepoints
  I find them clearer

• Use semantic markup, avoid echo
  echo's escape expansion is non-portable, using printf directly is preferable here. Or heredocs, as can be seen in some of the lengthier cases.

  See too: https://www.shellcheck.net/wiki/SC2028

• Use explicit error codes to distinguish fail modes
  This makes it easier for users to figure out what went wrong, plus adds documentation

• Make the prompts retry until correct input
  Also solidify them by using select loops where possible (which restricts the valid inputs)

• Use jq more extensively for output massaging

• Extract jq rendering code to use throughout
  These two I'm particularly proud of -- they simplify the comment selection code immensely.

• Matters of taste

  • "Status" updates are more accurately success reports
  • Reduce color, line weight -- current choice is very noisy
  • No need to remind us at the end which reaction was picked -- it should still be on screen
  • Remove broken char in comment selection prompt

  These are absolutely understandable if you reject, and just represent my personal preferences here

Closes: #2