test: wait for runtime readiness in integration tests (#4168)

Root cause:
The handoff test starts an agent immediately after workspace creation,
before the new shell is necessarily ready. The reconnect test assumes a
fixed sidebar row and searches raw ANSI output for text that can arrive
as separate incremental cell updates.

Fix:
Wait for a shell builtin to write a readiness marker before sending the
single raw agent.start request. Exercise the gap with a delayed shell.
Locate the recovered workspace in the rendered sidebar and check the
reconstructed screen for selection and input, including overlay borders.
Do not extend existing timeouts or add retries that hide failures.

Validation:
All 25 affected group and helper tests pass. Ten repeated rounds of the
two runtime tests and the row helper pass, for 30 test executions without
retries. Standalone native just ci passes 3582 Rust tests with six default
skips, formatting, all-target Clippy, 106 maintenance tests, six architecture
tests and 39 integration-asset tests. Windows cross-lint was not run because
its SDK is not installed.

Co-authored-by: JJ Liebig <jonathan.liebig@gmail.com>

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