Flips the read side of the cwd rearchitecture onto the _session_cwd
store introduced in the previous commit.
_authoritative_workspace_root now resolves:
1. the session's own cwd record (get_session_cwd) — per-session by
construction, so one session's cd can never leak into another
session's file resolution, with no ownership heuristics at all
2. registered override (fallback for cleared/never-written records)
3. legacy shared-env live cwd + preserved anchor (transition-only,
for commands that ran before this code loaded)
4. sentinel-free absolute TERMINAL_CWD
delegate_task children get their record seeded from the parent's at
spawn: they keep starting in the parent's directory (current behavior)
but their subsequent cds stay isolated in their own record instead of
bleeding back through the shared env.
The wrong-worktree leak class is now solved structurally on this path —
there is no shared cwd for sessions to inherit. The legacy env-side
tracking (cwd_owner, _live_cwd_if_owned, _last_known_cwd) remains only
as a transition fallback and is deleted in the next step.
ethernet
5461e0e098