Add content_hash(resource, *, algorithm='sha256') -> str to the
ResourceHandler protocol and all handler implementations:
- Protocol: new content_hash method on ResourceHandler (protocol.py)
- BaseResourceHandler: default impl hashes file content or directory
entry names; returns EMPTY_CONTENT_HASH sentinel for missing
resources
- GitCheckoutHandler: hashes git rev-parse HEAD through the requested
algorithm for consistent digest format
- FsDirectoryHandler: recursive walk hashing sorted relative paths
and file contents (content-only, ignores metadata)
- DevcontainerHandler: hashes devcontainer.json config file
- DatabaseResourceHandler: hashes connection string; for SQLite
file-based DBs, hashes the database file content
- _DefaultHandler: delegates to BaseResourceHandler
EMPTY_CONTENT_HASH sentinel is the SHA-256 of empty input
(e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855).
Hash algorithm is configurable via the algorithm parameter (default
sha256, accepts any hashlib.new()-compatible name).
Behave tests (10 scenarios): sentinel for missing/nonexistent,
determinism, different content produces different hash, fs-directory
recursive hash with change detection, git-checkout hash, configurable
algorithm (sha256 vs sha512), protocol compliance for all 4 handlers.
ISSUES CLOSED: #837
