Clean up stale session checks and stabilize test feedback

[dnouri]

Why

A maintenance review around #253 found unused session-verification scaffolding, stale filtered-test guidance, and a malformed Markdown fixture that made a GUI resize check intermittent.

This cleanup does not implement the watchdog proposed in #253.

Changes

• Remove the unused state-verification predicate, interval, timestamps, and isolated tests while preserving live state updates.
• Retain forward-compatible setWorkingMessage handling and document Pi RPC's current no-op behavior.
• Replace a stale filtered-test example and document ERT regexp escaping through Make.
• Terminate the GUI table fixture before filler text, scope overlay comparisons to the table, and verify filler receives no table overlays.
• Skip the resize assertion only when the window manager demonstrably refuses to narrow the chat window, and report ERT skips correctly in the GUI runner.

Verification

• make check — 1,642 tests, 0 unexpected
• Native PGTK GUI suite — 15 passed, 1 unsupported-resize skip
• X11 and xvfb GUI suites — 16/16 passed with the resize path exercised
• Focused resize test — reproduced 1/15 on master and 1/6 on feat: display inline images from tool results in chat buffer #221; 50/50 passed after fixing the fixture
• Selector check — one-backslash alternation ran 36 tests; the stale selector matched none