feat: support configurable Electron backend root - #153

Changes requested.

1. BLOCKER — backend/openbot/config.py:94-99: Root fallback detects whether overrides were explicit by comparing their parsed values to the defaults. This violates the stated “explicit WORKSPACE_ROOT and DATABASE_URL overrides remain authoritative” contract whenever an explicit override equals the default (for example WORKSPACE_ROOT=./workspace or DATABASE_URL=sqlite+aiosqlite:///./.openbot/openbot.db): the validator still replaces it with the supplied root. Use Pydantic's field-set/source information (or equivalent pre-validation) to distinguish omitted fields from explicitly supplied values, and add regression tests for both explicit-default cases.

2. NIT — Makefile:4: ELECTRON_ROOT_ARGS interpolates ROOT_DIRECTORY unquoted. A root path containing spaces is split into multiple positional arguments, so electron-dev.sh forwards only the first word as the root. Please either preserve argument boundaries (or document/reject such paths) and add a forwarding test if this option is intended to support normal filesystem paths.

— Reviewer - @Reviewer

Ready to merge: PR #153 has addressed the previously identified configuration override and Makefile argument-boundary issues. The updated diff preserves explicitly supplied WORKSPACE_ROOT/DATABASE_URL (including values equal to defaults), forwards root paths safely through development and packaged startup, and adds regression coverage. CI is green at the reviewed head f543d7206ec17b96ff4eb9287a038bd790c25425.

— Reviewer - @Reviewer

Ready to merge.\n\nRe-reviewed the exact head 63dca69. The cancellation race fix snapshots the task reference before the null check and cancels that stable reference; root-directory propagation, explicit override preservation, quoting, and regression coverage remain correct. CI is green.\n\n— Reviewer - @Reviewer