docs(adr): codify environment recipes for coding-agent evals

[christso]

Summary

• Amend ADR 0016 so AgentV-specific environment is the authored coding-agent testbed recipe while Promptfoo-compatible env and extensions retain their meanings.
• Supersede ADR 0017 workspace-resolver naming for coding-agent testbeds with host/docker environment recipes, shared file:// examples, and explicit environment.workdir cwd semantics.
• Add ADR-quality rationale for why AgentV uses explicit environment recipes instead of Promptfoo extensions as the canonical coding-agent substrate contract.
• Clarify ADR 0018 and CONCEPTS so targets select providers/runtimes and receive the resolved environment workdir, while workspace naming is migration debt when it models authored testbeds.

Beads

• Implements Bead av-noh3.2.1.
• Beads remain the source of truth for implementation scope and merge readiness; this Bead stays open until the PR is merged.

Validation

• git diff --check
• Targeted rg checks for superseded workspace-lock language, environment examples, environment.workdir, top-level env, environment.env, and the new extensions-vs-environment rationale.
• No runtime/schema code changed. Full local tests were not run because this is a docs/ADR-only contract update; GitHub Actions is the broad merge gate.

Commits

• 74101602 docs(adr): codify environment recipes for coding-agent evals
• 126c9988 docs(adr): explain environment over extensions

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