Add transcript backup tracking system and pre-compact hook improvements

- pre-compact-backup.sh: derive transcript path from session_id+cwd (no
  longer relies on transcript_path field that Claude Code stopped providing);
  register each backup in tracking.json after saving
- extract-transcripts.py: new script managing tracking.json — register,
  list, extract conversation text, mark-processed modes
- list-transcripts-here.sh: thin wrapper for extract-transcripts --list
  using $(pwd); needed because SKILL.md bang commands reject $() substitution
- install-hooks.sh: now also symlinks skill-helper scripts into
  ~/.claude/scripts/ via a curated SKILL_HELPERS list
- Memory docs: new script-extract-transcripts.md, script-list-transcripts-here.md;
  updated skill-log.md, script-install-hooks.md, MEMORY.md index

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/list-transcripts-here.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+# Wrapper for extract-transcripts.py --list, passing the current working directory.
+# Used by the /log skill's pre-gathered context (bang commands can't use $() substitution).
+python3 "$HOME/.claude/scripts/extract-transcripts.py" --list "$(pwd)"