emit budget_remaining metric to OTel

[cchinchilla-dev]

Summary

• Add OTel observable gauge agentloom_budget_remaining_usd in MetricsManager, following the same callback pattern as the circuit breaker gauge
• Update set_budget_remaining() to write to both OTel and Prometheus backends (was Prometheus-only)
• Add on_budget_remaining() hook to WorkflowObserver
• Wire engine to emit remaining budget after each step with cost
• Remove section separator comments (# ---) from observer.py, metrics.py, sandbox.py

Test plan

• uv run pytest — 690 pass, 7 e2e deselected
• uv run mypy src/ — no issues
• uv run ruff check src/ tests/ — all checks passed
• New tests for on_budget_remaining observer hook and _budget_remaining dict population

Closes #15

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull request overview

Adds first-class OpenTelemetry emission for “budget remaining” by extending the observability surface (metrics + observer hooks) and wiring the engine to publish remaining budget updates during workflow execution.

Changes:

• Add OTel observable gauge agentloom_budget_remaining_usd and store per-workflow remaining budget for callback-based export.
• Introduce WorkflowObserver.on_budget_remaining() and wire WorkflowEngine to emit remaining budget after each costed step.
• Update/extend tests to cover the new observer hook and _budget_remaining state updates.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/agentloom/observability/metrics.py	Adds _budget_remaining store and an OTel observable gauge callback; updates set_budget_remaining() to populate both OTel (dict) and Prometheus (gauge).
src/agentloom/observability/observer.py	Adds on_budget_remaining() hook to forward remaining budget to metrics.
src/agentloom/core/engine.py	Emits on_budget_remaining after each successful, costed step when a workflow budget is configured.
tests/observability/test_observer.py	Adds tests for the new observer hook behavior.
tests/observability/test_metrics.py	Adds assertion that set_budget_remaining() populates _budget_remaining.
src/agentloom/tools/sandbox.py	Removes section-separator comments only (no behavioral change).