feat(file-viewer): browse a versioned file's git history

A clock button in the file viewer header lists the versions of a file
whose project keeps a git history; picking one shows the file as of
that commit, read-only, with a banner back to the current version.

A past version is never served from the working tree: the whole
repository is materialized at that revision (git archive streamed
through tar into a size-bounded, immutable-by-rev cache) and every
fetch — content, compiled LaTeX, markdown images, downloads — resolves
inside that tree, so dependencies are contemporaneous with the file:
a .tex compiles against its \input's and images of that moment.

Backend: new git_versions module (repo discovery bounded by the
workspace mount, host-git log/rev-parse/archive, extraction cache with
oldest-first prune) + GET /api/file/versions and a rev param on
GET /api/file (rev is the ETag; never X-Writable). Frontend: history
mode in FileViewerBase shared by the desktop and mobile viewers —
popover, banner, watcher paused while browsing, rev propagated to
every /api/file URL it builds.

crates/skald-core/src/skald/bundles.rs

@@ -24,6 +24,7 @@ use crate::cron::TaskManager;
 use crate::elicitation::ElicitationManager;
 use crate::image_generate::ImageGeneratorManager;
 use crate::inbox::Inbox;
+use crate::git_versions::GitVersions;
 use crate::latex::LatexCompiler;
 use crate::llm::LlmManager;
 use crate::location::LocationManager;
@@ -440,6 +441,7 @@ impl Conversation {
 
 pub(super) struct Infra {
     pub(super) latex_compiler:   LatexCompiler,
+    pub(super) git_versions:     GitVersions,
     pub(super) location_manager: Arc<LocationManager>,
     pub(super) remote:           Arc<RwLock<Option<Arc<dyn RemoteAccess>>>>,
 }
@@ -448,6 +450,7 @@ impl Infra {
     pub(super) fn build() -> Self {
         Infra {
             latex_compiler:   LatexCompiler::new(),
+            git_versions:     GitVersions::new(),
             location_manager: Arc::new(LocationManager::new()),
             remote:           Arc::new(RwLock::new(None)),
         }