fix: preserve newest fusion context under byte budget

[leblancfg]

Problem

collectRecentConversation walks entries from newest to oldest, but it used to join the accumulated context and then truncate from the start when the byte budget was exceeded. If an older message pushed the joined string over budget, the truncation could remove the newest user/assistant exchange before the worker prompts were built.

That is backwards for fusion planning: workers need the latest turn more than stale context.

Change

• Track the byte budget while collecting conversation chunks.
• Stop adding older chunks once the next one would exceed the budget.
• Keep the existing truncation behavior only for the single-message case where the newest chunk is itself too large.
• Add a regression test that verifies the newest assistant message survives a small context budget.

Verification

• pnpm test -- tests/fusion.test.ts
• pnpm run typecheck
• pnpm run format:check
• git diff --check