Spell out the contract plugins will lean on: the context fields, the
"registry order, first non-null wins, throw = decline" rule, that compact
mode never consults providers, and that teardown is the contribution's
own disposer (nothing for ctx.onDispose). Include the exact migration for
compact-reasoning-label, noting that the core label no longer carries the
effort word, so the plugin's regex strip is already a no-op and the hook
is the place to compute a label rather than rewrite rendered text.
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