fix(zcode): re-add ZCode support with prompt-unwrap and resume-keep fixes

[26018gongyuan-boop]

Follow-up to #1040, which dropped ZCode because it could not be tested. ZCode 3.x
has now been debugged end-to-end on Windows with live hook instrumentation; the two
behaviors that made it look broken are fixed and live-verified, so this restores
ZCode as a supported host with those fixes on top.

What was wrong on ZCode

1. Slash commands arrive as markdown links. ZCode renders /ponytail ultra as
   [$ponytail](<skill path>) ultra in the UserPromptSubmit payload, so the
   tracker's ^[/@$]ponytail match never fired and level switches silently did
   nothing. The tracker now unwraps that form — gated on isZcode, so a
   [ponytail](url) link pasted at the start of an ordinary prompt on Claude
   Code / Codex / other hosts stays silent exactly as before.
2. A mid-session resume reset just-switched levels. ZCode restarts its runtime
   and resumes the session right after a slash-command prompt, firing
   SessionStart(resume) seconds after the tracker wrote the new level; activate's
   unconditional setMode(getDefaultMode()) then reset it to the default. activate
   now reads the hook payload's source and, on ZCode only (resume/compact), keeps
   the live flag. Other hosts keep their long-standing reset behavior — verified
   byte-identical per host and stdin shape against 552acd5.

Restored from #1040

isZcode detection (ZCODE_APP_VERSION), the strict-JSON output branch (#798), the
ZCode hook test, and the INSTALL.md / docs/agent-portability.md rows.

Notes

• activate now reads its stdin payload via the mode tracker's never-hang pattern
  (Bug: Claude Code freezes indefinitely on Windows after installing Ponytail #443/UserPromptSubmit hook still hangs to full timeout on Windows — #443 fix's setTimeout is unref'd #790) and exits only after stdout flushes (Windows pipe truncation guard);
  covered by a new self-exit test in tests/hooks-windows.test.js.
• Known limitation (pre-existing detection semantics, restored as-is): agents
  started from inside a ZCode terminal inherit ZCODE_APP_VERSION and are detected
  as ZCode.
• One pre-existing Windows-only test failure exists on main (Statusline badge breaks after a plugin update: the nudge points at the versioned plugin cache #1032 nudge
  assertion; fails identically on 552acd5, passes on Linux CI). Everything else
  passes: 127/128 locally including the restored + new ZCode cases, hooks-windows
  10/10, pi-extension 28/28.

[DietrichGebert]

Thanks! I removed ZCode in #1040 because I can't test it myself, and I only keep hosts I can verify.