fix(engine): deliver the final-step hint as a tail message, not a system append

[mgoldsborough]

Problem

On a run's last allowed iteration the engine appended a "this is your final step, wrap up" instruction to the system prompt:

if (iteration === maxIter - 1) callPrompt += "\n\n[IMPORTANT: This is your final step. ...]";

That mutates the cached system block. Since system is position 1 in Anthropic's tools → system → messages cache order, changing it invalidates the system breakpoint and the entire message prefix after it — so the final call of every run does a full-prefix re-write instead of reading the cached prefix. On a long conversation that's one expensive call per run.

Fix

Deliver the same hint as a tail message (the volatile 5-minute cache region from the TTL-tiering change), wrapped in <system-reminder>, leaving the stable system prefix byte-identical. The final call now reads the prefix from cache; only the small tail is new.

• Merges into a trailing user turn when the last message is already a user message (avoids consecutive-user role errors); otherwise appends a fresh user message.
• callPrompt is no longer mutated, so it's now const.

Tests

Updates the wrap-up test to assert the system prompt is byte-identical across all iterations (the cache-stability property) and the hint appears on the final turn's tail message, not the system block. Full unit suite (3,391) green; verify:static green.

Context

This is the last of the cheap cache-stability cleanups from the cost audit. The other candidate — pinning the compose.ts date — was dropped: it renders a date (toLocaleDateString), so it only busts the system block on conversations that span midnight (effectively never), and moving it properly belongs to the larger stable/volatile prompt partition (deferred until telemetry justifies it). The supervisor-tripped-tool mutation was also dropped — the forensics showed it wasn't a cost driver and gating it would weaken a safety mechanism.