Accept comment bodies via file/stdin and retire shell unescaping

[fiddur]

--body "..." forces agents to shell-escape, so unescape.ts guesses the escaping back out — and it is lossy by design: a comment discussing split('\n') gets a real newline.

Proposal: --body-file <path> / --body - (stdin) on comment/reply/amend/general-comment/tour commands; update the skills to prefer heredocs; retire fromShell once agents stop sending bodies through argv.

Effort: S.