Two fixes for the reported Slack thread approval UX:
1. Slack Block Kit approval/confirm sends silently overflowed the
3000-char section-block cap (flat 2900-char truncation + header +
reason), so long execute_code approvals failed with invalid_blocks
and fell back to the plain-text prompt with no buttons. Budget the
command preview against the rendered fixed parts so blocks never
exceed the cap (send_exec_approval + send_slash_confirm).
2. The text fallbacks told users to reply /approve — which Slack blocks
inside threads and Matrix clients reserve client-side. Add a
typed_command_prefix capability flag on BasePlatformAdapter
(default "/"; Slack and Matrix set "!" to match their existing
bang-prefix rewrite) and use it in the shared fallback prompt
builders (exec approval, update prompt, destructive slash confirm,
expensive-model confirm) plus Matrix's reaction-prompt text.
The slash-confirm text-intercept now also accepts bang-prefixed
replies (!always, !cancel) since those keywords aren't registered
commands and the adapters' rewrite doesn't touch them.
cd9a9cd8e5