feat(runtime): activate per-worktree session isolation (#41)

Remove #[cfg(test)] gate from session_control module — SessionStore
is now available at runtime, not just in tests. Export SessionStore and
add workspace_sessions_dir() helper that creates fingerprinted session
directories per workspace root.

This is the #41 kill shot: parallel opencode serve instances will use
separate session namespaces based on workspace fingerprint instead of
sharing a global ~/.local/share/opencode/ store.

The CLI already uses cwd/.claw/sessions/ (sessions_dir()), and now
SessionStore::from_cwd() adds workspace hash isolation on top.

rust/crates/runtime/src/lib.rs

@@ -35,8 +35,8 @@ pub mod recovery_recipes;
 mod remote;
 pub mod sandbox;
 mod session;
-#[cfg(test)]
-mod session_control;
+pub mod session_control;
+pub use session_control::SessionStore;
 mod sse;
 pub mod stale_base;
 pub mod stale_branch;

rust/crates/runtime/src/session.rs

@@ -1434,3 +1434,12 @@ mod tests {
             .collect()
     }
 }
+
+/// Per-worktree session isolation: returns a session directory namespaced
+/// by the workspace fingerprint of the given working directory.
+/// This prevents parallel `opencode serve` instances from colliding.
+pub fn workspace_sessions_dir(cwd: &std::path::Path) -> Result<std::path::PathBuf, SessionError> {
+    let store = crate::session_control::SessionStore::from_cwd(cwd)
+        .map_err(|e| SessionError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string())))?;
+    Ok(store.sessions_dir().to_path_buf())
+}