feat: workspace improvements - nullable cwd, repo-only workspaces, and resolution refactor

Make workspace cwd optional to support repo-only workspaces that don't require
a local directory. Refactor workspace resolution in heartbeat service to pass
all workspace hints to adapters, add fallback logic when project workspaces
have no valid local cwd, and improve workspace name derivation. Also adds limit
param to heartbeat runs list endpoint.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/migrations/0020_white_anita_blake.sql

@@ -0,0 +1 @@
+ALTER TABLE "project_workspaces" ALTER COLUMN "cwd" DROP NOT NULL;