feat(tui,webui): focus next session when current is deleted or archived

[edwin-zvs]

Summary

• TUI: adds focus_neighbor_of(id) — when the selected session is deleted or archived, the cursor moves to the adjacent session in the list (next, falling back to previous) instead of jumping to the first session or losing selection entirely.
• WebUI: same neighbor logic on session/deleted and session/state (archive); picks the next/previous session before clearing currentId, then calls selectSession(neighborId).

Both cases capture the neighbor before mutating state so the position is still valid.

Test plan

• cargo test -p agentd-cli — 275 tests pass
• TUI: with 3+ sessions, delete the middle one → focus moves to what was below it
• TUI: delete the last session in the list → focus moves to what was above it
• TUI: archive a session (C-x k → y/a) → focus moves to neighbor, archived session hidden
• WebUI: same flows via the toolbar delete/archive buttons