typescript: add git-credential-code-storage helper

[cloudhead]

Installs a Git credential helper bin that keeps token-free HTTPS remote URLs in Git config and mints a fresh short-lived JWT each time Git asks for credentials. Configure with:

git config credential.https://ORG.code.storage.helper code-storage
git config credential.https://ORG.code.storage.useHttpPath true

The helper reads PIERRE_PRIVATE_KEY_FILE or PIERRE_PRIVATE_KEY and PIERRE_TOKEN_TTL (default 1 hour).

> export PIERRE_PRIVATE_KEY_FILE=$HOME/.pierre/test.api.key

> git remote -v                                                                         
origin    https://pierre.code.storage/19ff8064-f66d-4b05-86a0-9a2e54b34ae6.git (fetch)
origin    https://pierre.code.storage/19ff8064-f66d-4b05-86a0-9a2e54b34ae6.git (push)

> which git-credential-code-storage 
/home/cloudhead/.npm/bin/git-credential-code-storage

> git ls-remote origin
6be2d8a3c23620bd1f4824f9932dbd5ce00f96f8    refs/heads/master

With debug:

> PIERRE_DEBUG=1 git ls-remote origin
git-credential-code-storage: signing with private key from /home/cloudhead/.pierre/test.api.key
git-credential-code-storage: acquiring git:write+git:read token for repo 19ff8064-f66d-4b05-86a0-9a2e54b34ae6 (org pierre, ttl 3600s)
git-credential-code-storage: token acquired for pierre.code.storage/19ff8064-f66d-4b05-86a0-9a2e54b34ae6.git
git-credential-code-storage: ignoring 'store' operation: only 'get' mints credentials
6be2d8a3c23620bd1f4824f9932dbd5ce00f96f8    refs/heads/master