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>

memory/script-list-transcripts-here.md

@@ -0,0 +1,26 @@
+# script: list-transcripts-here
+
+**Location:** `claude-foundations/scripts/list-transcripts-here.sh`  
+**Symlinked to:** `~/.claude/scripts/list-transcripts-here.sh`
+
+## Purpose
+
+Thin wrapper around `extract-transcripts.py --list "$(pwd)"`. Exists because SKILL.md bang commands (`!`command``) cannot use `$()` substitution — the Claude Code permission checker rejects it. The wrapper runs the substitution internally (in bash, where it's fine) and outputs the result.
+
+## Usage
+
+```bash
+# From within a project directory
+bash ~/.claude/scripts/list-transcripts-here.sh
+# → JSON array of unprocessed transcript backups for the current cwd
+```
+
+## How it works
+
+Single line: `python3 "$HOME/.claude/scripts/extract-transcripts.py" --list "$(pwd)"`
+
+## Gotchas
+
+- Must be called with `bash list-transcripts-here.sh` (not `python3`) — it's a shell wrapper, not a Python script
+- Output is the same as `extract-transcripts.py --list`, so check that script's docs for the JSON format
+- The `$()` substitution is the entire reason this wrapper exists; if that restriction is ever lifted from SKILL.md bang commands, this wrapper can be removed