fix(agent): focus agents by pane id

[dmissoh]

Fixes #18.

Agent rows never focus: FocusAgent targets terminal_id, but herdr resolves agent targets by pane_id only.

herdr agent get term_65748533b89a71  -> {"error":{"code":"agent_not_found", ...}}
herdr agent get w653fdbd6bc7b02:p2D  -> ok, same agent

terminal_id is always present in herdr agent list output, so the existing unwrap_or(pane) fallback never fires and every agent row is affected.

Change

Focus by pane_id, falling back to terminal_id if pane_id is ever absent. terminal_id stays in search_terms, so searching an agent by its terminal id still matches.

Verification

• Repro before the patch, agent focus works after it. herdr 0.7.5, macOS arm64.
• cargo test: 65 pass, 2 failures (close_target_matches_entry_kind, source_specific_reuse_distinguishes_same_path_workspaces) that also fail on unmodified main — both query a live herdr server.
• cargo fmt --check clean. cargo clippy --all-targets unchanged: one pre-existing items after a test module warning in src/main.rs.