fix(relay): auto-create git_repo_path so kind:30617 init can't silently fail

[tlongwell-block]

Problem

The git smart-HTTP transport and the kind:30617 side-effect handler both canonicalize config.git_repo_path before any disk work. If that directory doesn't exist on the relay host, two things happen and neither is obvious:

1. Transport — every clone/push gets HTTP 500 git service misconfigured (crates/sprout-relay/src/api/git/transport.rs:237).
2. Side effect — handle_git_repo_announcement fails at canonicalize_root(). The ingest pipeline only logs side-effect failures at warn! level (ingest.rs:1530), so the kind:30617 event is stored but the bare repo is never created on disk. From the client's perspective, the relay accepted the announcement (you can get-repo it back), but every subsequent push 500s.

Reproduced on stage: published a valid kind:30617 from the CLI, got a successful event ID back, then git push immediately hit git service misconfigured.

This currently requires operators to mkdir the directory out of band. Since the relay already owns the data layout below that root ({owner_hex}/{repo}.git/, .names/...), it should self-provision the root the same way it does for other relay-owned state.

Fix

• config.rs — create_dir_all(git_repo_path) during config load. Fails loudly with ConfigError::InvalidValue if the path can't be created (permissions, path-under-a-file, etc.), instead of letting the relay come up healthy with a silently broken git service.
• side_effects.rs::handle_git_repo_announcement — defensive create_dir_all(git_repo_root) before canonicalize, in case the directory is removed at runtime or a future deployment path bypasses config bootstrap. Returns a clear error rather than the cryptic failed to canonicalize repo root you'd otherwise see in logs.

Tests

• git_repo_path_is_created_if_missing — config load against a non-existent nested path succeeds and creates the directory.
• git_repo_path_unwritable_returns_error (unix-gated) — config load against a path under /dev/null returns ConfigError::InvalidValue mentioning SPROUT_GIT_REPO_PATH.

running 7 tests
test config::tests::git_repo_path_is_created_if_missing ... ok
test config::tests::git_repo_path_unwritable_returns_error ... ok
... (146 lib tests pass, 0 fail)

Post-deploy note

Existing kind:30617 announcements published against a broken-root relay won't auto-heal — their side effect already ran (and was swallowed by warn!). Owners need to re-publish the announcement (a fresh event with bumped created_at) to trigger the init path. New announcements after deploy are fine.

Scope

Relay-only change. No API/wire/format changes. No client changes needed.