hooks: extend stdin-EOF fix to the readers that bypass the shared helper

Self-review found the first commit only fixed agents that route through ReadAndParseHookInput. Four readers had their own io.ReadAll(stdin) and so kept the #1398 hang: copilot (readHookEnvelope), pi and external (ParseHookEvent), and the claude-code post-todo path (parseSubagentCheckpointHookInput).

Extract the guard + streaming decode into a shared primitive (ReadHookInputRaw) plus StdinLooksInteractive, and wire the bypassers to it:

• ReadAndParseHookInput now builds on ReadHookInputRaw.
• copilot uses ReadHookInputRaw (its parseHookEnvelope needs the raw bytes for key-name fallbacks).
• pi uses ReadAndParseHookInput[piHookPayload].
• claude-code post-todo uses ReadAndParseHookInput.
• external forwards raw stdin bytes verbatim to its subprocess (may be empty/non-JSON), so it keeps io.ReadAll but adds the interactive-terminal guard; the piped-held-open case for external plugins is documented as a residual.