Resumable launcher silently records dispatcher_commit: null from a sandbox checkout

## Context

`launch_beaker_resumable.py` wrote `"dispatcher_commit": null` into the launch record for a
launch made from the Claude Science Mac sandbox (#107), with no warning on stdout. The rest
of the record was correct, and all three runtime REFs pinned to full SHAs normally.

## Root cause

`_git_sha()` (`code/launch_beaker_resumable.py:70-82`) shells out to
`git -C rev-parse HEAD` and swallows every failure into `return None`:

```python
except Exception:
pass
return None
```

The git-session-isolation route the sandbox is required to use puts the repository's
git-dir at a path **not** named `.git` (the sandbox blocks creating any such path), so a
bare `git -C rev-parse HEAD` finds no repository and exits non-zero:

```
fatal: not a git repository (or any of the parent directories): .git
```

Verified both directions on the same checkout: with `GIT_DIR`/`GIT_WORK_TREE` exported the
call returns `d49b4a8…`; without them it fails as above and the record silently gets `null`.

## Why it matters

The launch record is the artifact that says which dispatcher code composed a launch.
`dispatcher_commit: null` is a provenance hole that appears exactly on the documented
sandbox launch path, and it is silent — the launcher prints the record and reports success.
It is also recoverable in practice (`DISPATCHER_REF` in the same record carries a resolved
SHA), which is why it went unnoticed.

## Done when

- [ ] `_git_sha` failure is surfaced, not swallowed — warn on stderr when it returns None
- [ ] The launcher records a dispatcher SHA when run from an external-git-dir checkout
- [ ] `references/sandbox-launch.md` states the `GIT_DIR` requirement, or the launcher
stops needing it

## Notes

Workaround in use meanwhile: export `GIT_DIR`/`GIT_WORK_TREE` before invoking the launcher.
A code fix could prefer `git rev-parse` in the ambient environment before falling back to
`-C`, since the env vars are already set correctly by the isolation helper.

Verified at dispatcher `d49b4a8`.