feat(hosts): add redacted execution preview contract

[mrnicholasbcarter-code]

Summary

• add the versioned execution-host/v1 contract boundary
• define Codex, Claude Code, and Pi host identities and lifecycle states
• require deterministic, redacted execution previews with budgets, permissions, fan-out, and objective digests
• validate declared adapter capabilities and truthful terminal results
• document the boundary and add focused tests

Closes #108

Validation

• focused execution-host tests: 7 passed
• Ruff: passed
• strict mypy: passed
• pre-commit hook: passed

[github-actions]

📊 Benchmark Results

coding

Metric	Fixed	Verdict	Delta
Success Rate	96.0%	97.5%	1.5%
Cost/req	$0.014862	$0.006943	53.3%
p95 Latency	148ms	141ms	-7ms

reasoning

Metric	Fixed	Verdict	Delta
Success Rate	100.0%	97.5%	-2.5%
Cost/req	$0.007620	$0.004675	38.6%
p95 Latency	144ms	145ms	1ms

chat

Metric	Fixed	Verdict	Delta
Success Rate	98.0%	99.0%	1.0%
Cost/req	$0.000106	$0.001424	-1243.4%
p95 Latency	73ms	144ms	70ms