Replaces the bespoke parent-only disambiguation in MentionUri::disambiguated_name with the shared util::disambiguate::compute_disambiguation_details utility — the same one that drives buffer tab titles, the sidebar, and the recent projects picker.

Why

The current logic only ever appends one parent directory, so two files like /a/src/foo.rs and /b/src/foo.rs both render as src/foo.rs — still ambiguous. The shared utility iteratively bumps a per-item detail level until no two labels collide, handling arbitrary depth and terminating safely at fixed points.

Changes

• MentionUri::disambiguated_name(needs_disambiguation: bool) → disambiguated_name(detail: usize). File/Directory now delegates to project::path_suffix(abs_path, detail). Skill reaches its fixed point at detail >= 1; other variants are fixed points at detail 0.
• compute_disambiguated_labels in mention_set.rs feeds all mentions into compute_disambiguation_details and maps each to its label at the computed level. All variants are disambiguated against each other (matches what the user sees in the flat list of pills, and matches pane.rs::tab_details).

Tests

• Added multi-level coverage to test_disambiguated_name for deep file collisions and fixed-point behavior on Thread.
• All 31 acp_thread mention tests pass; agent_ui builds clean.

Release Notes:

• N/A