refactor(#2791): extract seedMessages/BudgetTracker/handleToolCalls — loop is now orchestration

Slice 2 (final) of decomposing the executeAgentic god-function. Slice 1 (#188)
shipped snapshotOf + the exhaustive BudgetDecision when; this completes the
remaining acceptance, behavior-preserving:

- BudgetTracker (new) — the invocation's mutable budget state (turns/toolCalls/
  totalTokens + effective caps + consecutive-tool counter + #966 threshold
  bookkeeping), the Stop/Extend/Checkpoint cap dispatch (resolveCapDecision), and
  the one SessionSnapshot builder. Counters that were ~15 loose `var`s + 3 local
  closures now live in one testable class.
- seedMessages (new) — the fresh-run message seeding (system prompt + custom
  cache segments + user message + image-attachment dereference), symmetric to the
  existing restoreFromSnapshot.
- handleToolCalls (new, ToolCallHandling.kt) — the ~185-line LlmResponse.ToolCalls
  arm, further split into dispatchToolCall / handleUnknownTool / emitToolOutcome /
  buildInterrupt so each unit stays well under the complexity thresholds.
- accumulateUsage (new) — the cumulative TokenUsage merge, named.

executeAgentic loop file: 739 -> 352 lines; the loop body reads as orchestration.
No behavior change — every counter, listener, audit event, and ordering is
preserved (full suite green). New BudgetTrackerTest (9) unit-tests the lifted
state directly; LongParameterList stays off so the wide internal seams are fine.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Skobeltsyn