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>

tests/hooks/test-session-start.sh

@@ -4,7 +4,6 @@ set -euo pipefail
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
 REPO_ROOT="$(cd "$SCRIPT_DIR/../.." && pwd)"
 HOOK_UNDER_TEST="$REPO_ROOT/hooks/session-start"
-CODEX_HOOK_UNDER_TEST="$REPO_ROOT/hooks/session-start-codex"
 WRAPPER_UNDER_TEST="$REPO_ROOT/hooks/run-hook.cmd"
 
 FAILURES=0
@@ -154,35 +153,15 @@ assert_command_output \
     CLAUDE_PLUGIN_ROOT="$REPO_ROOT" \
     bash "$HOOK_UNDER_TEST"
 
-codex_home="$(make_home codex-plugin-hooks)"
-codex_data="$TEST_ROOT/codex-plugin-hooks/data"
-mkdir -p "$codex_data"
+wrapper_home="$(make_home run-hook-wrapper)"
 assert_command_output \
-    "Codex plugin hooks use dedicated script and emit nested SessionStart additionalContext" \
+    "run-hook.cmd wrapper dispatches to the named session-start script" \
     "nested" \
     "" \
     "" \
-    "$codex_home" \
-    PLUGIN_DATA="$codex_data" \
-    CLAUDE_PLUGIN_DATA="$codex_data" \
-    PLUGIN_ROOT="$REPO_ROOT" \
+    "$wrapper_home" \
     CLAUDE_PLUGIN_ROOT="$REPO_ROOT" \
-    bash "$CODEX_HOOK_UNDER_TEST"
-
-codex_wrapper_home="$(make_home codex-wrapper)"
-codex_wrapper_data="$TEST_ROOT/codex-wrapper/data"
-mkdir -p "$codex_wrapper_data"
-assert_command_output \
-    "Codex wrapper path dispatches to dedicated script" \
-    "nested" \
-    "" \
-    "" \
-    "$codex_wrapper_home" \
-    PLUGIN_DATA="$codex_wrapper_data" \
-    CLAUDE_PLUGIN_DATA="$codex_wrapper_data" \
-    PLUGIN_ROOT="$REPO_ROOT" \
-    CLAUDE_PLUGIN_ROOT="$REPO_ROOT" \
-    bash "$WRAPPER_UNDER_TEST" session-start-codex
+    bash "$WRAPPER_UNDER_TEST" session-start
 
 cursor_home="$(make_home cursor)"
 assert_command_output \
@@ -217,21 +196,6 @@ assert_command_output \
     CLAUDE_PLUGIN_ROOT="$REPO_ROOT" \
     bash "$HOOK_UNDER_TEST"
 
-codex_legacy_home="$(make_home codex-legacy-warning-removed)"
-codex_legacy_data="$TEST_ROOT/codex-legacy-warning-removed/data"
-mkdir -p "$codex_legacy_home/.config/superpowers/skills" "$codex_legacy_data"
-assert_command_output \
-    "Codex SessionStart omits obsolete legacy custom-skill warning" \
-    "nested" \
-    "" \
-    "Superpowers now uses"$'\037'"~/.config/superpowers/skills"$'\037'"~/.claude/skills"$'\037'"legacy" \
-    "$codex_legacy_home" \
-    PLUGIN_DATA="$codex_legacy_data" \
-    CLAUDE_PLUGIN_DATA="$codex_legacy_data" \
-    PLUGIN_ROOT="$REPO_ROOT" \
-    CLAUDE_PLUGIN_ROOT="$REPO_ROOT" \
-    bash "$CODEX_HOOK_UNDER_TEST"
-
 if [[ "$FAILURES" -gt 0 ]]; then
     echo "STATUS: FAILED ($FAILURES failure(s))"
     exit 1