fix(telegram): resolve send_attachment paths in the user's workspace

`send_attachment` handed its `file_path` argument straight to
`InputFile::file`, which resolves against the **server process's** working
directory. Every path the model can actually have — relative to the user's
home, or absolute inside their container — failed the `path.exists()` check,
and the one class that didn't (a name that happens to exist next to the
binary) would have sent the wrong file.

The routing already exists for the fs-tools, so expose it rather than repeat
it: `UserFilesApi` (core-api) reads a path in the agent's own vocabulary and
is obtained from `UserChannelHandle::files()`, so it is scoped to one user by
construction. skald-core implements it over `resolve_view_target` — host
mount read directly, container-only path through `docker exec` — holding the
`SharedFs` cell rather than a snapshot, so a remount lands without a login.

The size cap is checked before the read (a new `exec_fs::size` for the
container branch): the point of a cap is to keep an oversized file out of RAM,
so checking it afterwards would protect nothing. A photo above `sendPhoto`'s
narrower 10 MB ceiling goes out as a document instead of as an API error.

crates/skald-core/src/container/exec_fs.rs

@@ -93,6 +93,20 @@ pub async fn write(container: &str, path: &Path, bytes: &[u8]) -> Result<()> {
     Ok(())
 }
 
+/// Byte size of a file inside the container — for the callers that must decide
+/// whether to read it *before* pulling it through the pipe. `wc -c` rather than
+/// `stat`, so the answer is the same on any of the image's shells.
+pub async fn size(container: &str, path: &Path) -> Result<u64> {
+    let p = path.to_string_lossy();
+    let raw = sh(container, r#"wc -c < "$1""#, &[&p])
+        .await
+        .with_context(|| format!("Cannot stat file: {p}"))?;
+    String::from_utf8_lossy(&raw)
+        .trim()
+        .parse()
+        .with_context(|| format!("Cannot stat file: {p}"))
+}
+
 pub async fn exists(container: &str, path: &Path) -> bool {
     sh_ok(container, r#"test -e "$1""#, &[&path.to_string_lossy()]).await
 }