harness: fix py-vs-python3 doc gap — add py.sh resolver, repoint skill/command docs

The skill/command DOCS instructed Claude to run a bare `py ...`, which is the
Windows py-launcher — absent on Linux/macOS (exit 127, hit on GURU-KALI). A blind
py->python3 swap is wrong too: python3 is a broken MS Store shim on some Windows
boxes where `py` is the correct launcher.

Fix mirrors the resolution the .sh skill scripts already do:
- New .claude/scripts/py.sh: picks the interpreter that actually RUNS —
  identity.json python.command first, then py -> python3 -> python, each
  validated with `-c 'import sys'` so the MS Store stub is skipped. exec's it.
- Repointed all DOC invocations (10 files, ~70 sites) from `py ...` to
  `bash "$CLAUDETOOLS_ROOT/.claude/scripts/py.sh" ...` (incl. the `py -c` and
  `py -` heredoc forms in checkpoint.md / mailbox.md).
- Left the .sh skill scripts untouched — they already resolve py/python/python3.
- errorlog.md: marked the GURU-KALI entry RESOLVED.

Depends on CLAUDETOOLS_ROOT (seeded by ensure-settings-env.py); py.sh also
self-resolves the repo root via git/cwd as a fallback.

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

.claude/skills/b2/SKILL.md

@@ -25,9 +25,9 @@ work with `python`/`python3`.
 
 ```bash
 # from the scripts dir, or pass full paths
-py "$CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py" status
-py "$CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py" buckets
-py "$CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py" usage --json
+bash "$CLAUDETOOLS_ROOT/.claude/scripts/py.sh" "$CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py" status
+bash "$CLAUDETOOLS_ROOT/.claude/scripts/py.sh" "$CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py" buckets
+bash "$CLAUDETOOLS_ROOT/.claude/scripts/py.sh" "$CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py" usage --json
 ```
 
 Transport auto-selects: uses `httpx` if installed, otherwise stdlib `urllib`
@@ -95,7 +95,7 @@ into tickets/logs. Destructive calls are NEVER retried automatically.
 ## Common commands
 
 ```bash
-B2="py $CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py"
+B2="bash "$CLAUDETOOLS_ROOT/.claude/scripts/py.sh" $CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py"
 
 # Status / inventory
 $B2 status
@@ -175,7 +175,7 @@ that would exceed that.
 ### Commands
 
 ```bash
-B2="py $CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py"
+B2="bash "$CLAUDETOOLS_ROOT/.claude/scripts/py.sh" $CLAUDETOOLS_ROOT/.claude/skills/b2/scripts/b2.py"
 
 # 1. READ-ONLY: see the bucket's current lifecycle rules + revision
 $B2 lifecycle ACG-Internal