#4222: fix: canvas root should respect agents.defaults.workspace

## Summary When `canvasHost.root` is not explicitly configured, the canvas directory defaults to `~/clawd/canvas` regardless of the user's `agents.defaults.workspace` setting. This PR derives the default from the workspace directory instead, so users with a custom workspace (e.g. `~/Bot`) get `~/Bot/canvas` automatically. Closes #2282 - Pass `defaultWorkspaceDir` into `createGatewayRuntimeState` - Use `path.join(defaultWorkspaceDir, "canvas")` as fallback when `canvasHost.root` is unset - Update doc comment in `CanvasHostConfig` to reflect the new default Backward compatible: if workspace is not customized, the default remains `~/clawd/canvas`. ## Test plan - Set `agents.defaults.workspace` to a custom dir (e.g. `~/Bot`), leave `canvasHost.root` unset → canvas mounts at `~/Bot/canvas` - Explicitly set `canvasHost.root` → that value is used (unchanged behavior) - No workspace or canvas config → defaults to `~/clawd/canvas` (unchanged behavior) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change threads the resolved default agent workspace directory (`defaultWorkspaceDir`) into gateway runtime initialization and uses it to derive the canvas host root when `cfg.canvasHost.root` is unset. Specifically, `createGatewayRuntimeState` now falls back to `path.join(defaultWorkspaceDir, "canvas")` rather than an implicit hardcoded workspace path, so users customizing `agents.defaults.workspace` get a matching default canvas directory. The behavior for an explicitly configured `canvasHost.root` remains unchanged; only the implicit default changes. <h3>Confidence Score: 4/5</h3> - This PR is low risk and changes a single default path derivation without altering explicit configuration behavior. - Changes are localized to gateway startup/runtime wiring and a nullish-coalescing fallback for `canvasHost.root`; the main risk is around documentation clarity and how “workspace” is defined (default agent vs per-agent) rather than runtime correctness. - src/config/types.gateway.ts (comment accuracy) <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->