feat(tui): turn relay into a stable session workbench

[ZhiXiao-Lin]

Summary

• give native sessions and external transcripts stable identities so refresh reordering and appended transcript metadata do not move selection to a different task
• preserve an independent selection for each A3S Code, Claude Code, Codex, and WorkBuddy tab
• add multi-term search across task, status, model, session id, and source path
• add bounded 64-row catalogs, globally budgeted transcript scans, preview, wheel/Home/End/Page navigation, manual refresh, and generation-scoped 15-second refresh
• pin the current native session and surface persisted state, model, age, unfinished runs, and live background-agent counts
• preserve the last good catalog when a refresh fails or an obsolete scan finishes late

Validation

• cargo fmt --all -- --check
• cargo test --bin a3s relay — 19/19 passed
• cargo check --bin a3s
• git diff --check

Recovery scope

This is the focused extraction of the completed /relay work from the preserved live CLI session. It intentionally excludes the subsequent /tasks work, DeepResearch changes, execution-mode work in #32, and session fork/rewind work.

[ZhiXiao-Lin]

Integrated through #40 at merge commit c252583e413f70a4cb265012d776f9d050b94de5. The consolidated v0.9.7 branch contains this PR's latest head 187f3560a6e32d4909f57a6fc5379eaa285f2abb and passed Linux, macOS, and Windows CI.