feat: warn the agent at 80% of a run budget before hard stop

[salignatmoandal]

Summary

The budget already existed. The agent never saw it until the run was killed.

• Warn once at 80% of max_cost / max_tokens / max_time, then keep going.
• On the next turn the model gets a system message so it can cheapen tools, summarize, or finish instead of slamming into the hard stop.
• The budget_exceeded JSON contract and kill-switch at 100% are unchanged.

Why this is interesting

enforceBudget was a fuse, not a policy. The runtime already counted cost, tokens, and time (record / snapshot / per-agent spend), but the model only learned the ceiling after iterationStop. From the user's point of view that looks like a crash: Execution stopped after reaching budget.max_cost.

That is the EcoAgent-Bench gap: completing under a budget is not the same as choosing economical actions. A kill-switch with no feedback trains the agent to spend until the wall.

This PR turns the counters into a signal inside the loop — a change in agency policy, not an extra log line.

Before	After
Budget = kill-switch	Budget = signal + kill-switch
Agent is blind until death	Agent can still adapt
Counters unused by the model	Counters feed the next prompt

What that buys docker-agent:

1. Fewer runs cut mid-task (code, MCP, delegation).
2. Less waste: the agent can stop repeating tools or finish sooner.
3. A reusable primitive: the runtime can now talk to the model from a metric. Soft budget today; later, model routing or denying expensive tools.

What it does not claim:

• It does not pick cheaper tools by itself — it gives the LLM the information.
• It does not add YAML (80% is an internal constant, no schema bump).
• It does not relax the ceiling. Ignore the warning and the 100% stop still fires.

Behavior

At the existing turn-boundary check in RunStream:

1. If any ceiling is hit → same as today (budget_exceeded + assistant stop message + iterationStop).
2. Else if a limit is in [80%, 100%) and that tracker has not warned for that limit yet:
   • emit the existing Warning event (TUI / JSON stream)
   • append a system message the model will see on the next GetMessages call
3. Each tracker warns once per limit (cost, then tokens, then time — same priority as exceeded()).
4. Unpriced spend still does not invent a cost warning (the existing unpriced Warning remains).

Test plan

• go test ./pkg/runtime -count=1 -run 'TestBudget|TestNilBudget|TestEnforceBudget|TestRecordBudget'
• go test ./pkg/runtime -count=1 -run 'TestBudgetExceededEventJSONContract|TestEnforceBudgetWarnsOnceThenStillHardStops'
• task test
• task lint

[aheritier]

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