refactor: extract report-renderer into agent-reliability-core — ADR-4 complete

[MarouaBoud]

Summary

Extracts the last shared primitive — report-renderer (ADR-4) — completing the agent-reliability-core extraction. Implements the seam from the design note (#19), approved as closed section set + [render] extra.

The seam

Rendering is split into a domain-agnostic presentation model in core and a per-tool adapter that stays in each tool:

RunReport ──to_report()──▶ core.report.Report (Badge + KpiRow/Table/Callout/Note) ──▶ HTML / terminal

• core agent_reliability_core.report: the closed section vocabulary, the oxblood Theme (tone→hex + tone→Rich style), and both renderers. HTML/terminal backends are the opt-in [render] extra (jinja2+rich); the model + theme are dependency-free.
• stampede: RunReport/build_report/to_dict unchanged (JSON contract + determinism intact). New observer/report_view.to_report() adapter; renderer.py is now two delegating functions with identical signatures; the stampede template is deleted.

Verified

• 131 tests pass (incl. 3 standalone core render tests importing only agent_reliability_core.report, and a stampede adapter test).
• Visual parity checked: rendered HTML still carries every section, the grade badge, the confusion bars, and the #6b1f2a oxblood token; theme is overridable.
• ruff + mypy clean on both packages; core wheel bundles the generic template.

ADR-4 complete

All four primitives are now extracted: trace-format, concurrency-core, persona-pack, report-renderer. agent-reliability-core is a real, independently-tested distribution the siblings (mcp-probe, costbomb, mockworld) can consume.