chore(codex): remove orphaned session-start-codex hook + refresh hook docs

hooks/session-start-codex has had no caller since "Remove Codex hooks"
(#1845) deleted hooks-codex.json and its manifest registration; the
Codex manifest now declares an empty hooks object so Codex registers no
session-start hook at all. The script is Codex-specific dead code —
nothing executes it on Codex or any other harness.

- Delete hooks/session-start-codex.
- tests/hooks/test-session-start.sh: drop the two Codex cases that are
  redundant with the generic session-start tests (nested-format and the
  legacy-warning omission are already covered by the Claude Code cases).
  Re-point the "wrapper dispatches" case to the live `session-start`
  script so run-hook.cmd dispatch coverage — used by Claude Code and
  Cursor in production — is preserved rather than lost.
- docs/porting-to-a-new-harness.md: Codex is no longer a Shape A
  (shell-hook) harness, so re-anchor that worked example to Cursor (a
  live shell-hook harness that demonstrates the same per-harness field,
  schema, and matcher variance) and mark Codex as native skill discovery
  with no session-start hook. Clears the references to the deleted
  hooks-codex.json.
- docs/windows/polyglot-hooks.md: the "check hooks-codex.json" pointer
  referenced a file deleted in #1845; re-point to hooks-cursor.json.

RELEASE-NOTES.md keeps its historical mention of hooks-codex.json (it
accurately records what that release did). The tests/codex-plugin-sync
fixtures build their own synthetic session-start-codex and test the sync
mechanism generically, so they are intentionally left as-is.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hooks/session-start-codex

@@ -1,26 +0,0 @@
-#!/usr/bin/env bash
-# Codex SessionStart hook for superpowers plugin
-
-set -euo pipefail
-
-SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
-PLUGIN_ROOT="$(cd "${SCRIPT_DIR}/.." && pwd)"
-
-using_superpowers_content=$(cat "${PLUGIN_ROOT}/skills/using-superpowers/SKILL.md" 2>&1 || echo "Error reading using-superpowers skill")
-
-escape_for_json() {
-    local s="$1"
-    s="${s//\\/\\\\}"
-    s="${s//\"/\\\"}"
-    s="${s//$'\n'/\\n}"
-    s="${s//$'\r'/\\r}"
-    s="${s//$'\t'/\\t}"
-    printf '%s' "$s"
-}
-
-using_superpowers_escaped=$(escape_for_json "$using_superpowers_content")
-session_context="<EXTREMELY_IMPORTANT>\nYou have superpowers.\n\n**Below is the full content of your 'superpowers:using-superpowers' skill - your introduction to using skills. For all other skills, follow the Codex skill-loading instructions in that skill:**\n\n${using_superpowers_escaped}\n</EXTREMELY_IMPORTANT>"
-
-printf '{\n  "hookSpecificOutput": {\n    "hookEventName": "SessionStart",\n    "additionalContext": "%s"\n  }\n}\n' "$session_context" | cat
-
-exit 0