Wrap background completion output in <background_output> XML tags with
inline instructions to treat as data, consistent with <user_input> tags
used for user steering hints.
Guard listener iteration in InjectionService.addInjection and
ExecutionLifecycleService.settleExecution with try/catch so a throwing
listener doesn't block subsequent listeners or crash the caller.
Rename getUserHints/getUserHintsAfter/getLatestHintIndex to
getInjections/getInjectionsAfter/getLatestInjectionIndex with optional
source filter so bg completions don't get formatted as user hints.
Swap unshift ordering so bg completions appear before user hints in the
message — the model sees context before the user's reaction to it.
Remove unused getLastUserHintAt().
Remove legacy onUserHint/offUserHint/addUserHint methods. All callers
now use addInjection(text, source) and onInjection/offInjection with
source-based filtering where needed.
Rename UserHintService to InjectionService as a generic, source-agnostic
injection mechanism. InjectionService supports typed sources ('user_steering'
and 'background_completion') with source-specific gating — user_steering
respects the model steering toggle while background_completion always fires.
Add background completion lifecycle to ExecutionLifecycleService: tracks
backgrounded executions, fires onBackgroundComplete listeners when they
settle, and supports FormatInjectionFn callbacks so execution creators
control how their output is formatted for reinjection.
Wire AppContainer to route background completions through InjectionService
and submit them to the model when idle, independent of model steering.
Adam Weidman