* fix(backend): inherit parent workspace for subtasks on all start paths
Subtasks with workspace_mode inherit_parent were provisioning a fresh
worktree and tasks folder instead of reusing the parent's. Two causes:
- propagateInheritedEnvironment ran only in PrepareTaskSession, so the
direct start path (startTask, used by MCP auto-start) never bound the
child session to the parent's TaskEnvironment. Moved it into
prepareSessionForStart so every launch entry point inherits.
- The service CreateTask path did not inherit the parent's repositories
when none were supplied (the UI omits them expecting inheritance), so
the child was repo-less and could not establish/reuse a worktree. Now
mirrors the MCP path: inherit RepositoryID + BaseBranch, drop
CheckoutBranch.
* fix(web): hide executor profile for inherit-parent subtasks
When a subtask inherits the parent's materialized workspace, its
executor is inherited too, so picking an executor profile is
meaningless. Hide the selector under inherit_parent and omit
executor_profile_id from the create payload.
* fix(backend): fail fast when subtask parent-repo inheritance lookup errors
Return the error from inheritParentRepositories instead of swallowing it:
a subtask silently created with no repositories would reintroduce the
fresh-worktree bug at launch. Also clarified that inheritance applies to
any repo-less subtask, matching the MCP path.
Carlos Florêncio