chore: retire src/eval.ts and npm run eval

[dhruva-reddy]

Summary

• Delete src/eval.ts and the npm run eval script. Both have been dead-code-walking since Stack E shipped npm run sim (improvements.md entry feat: push --dry-run preview mode #16, resolved 2026-04-30). The two runners coexisting under similar names actively misled engineers and customer forks.
• Scrub all doc references: README.md (commands table row, Direct-Mode example block, the ### Running Evals section, the src/ directory-tree comment) and AGENTS.md (testing-commands block).
• Rewrite the two back-reference comments in src/sim.ts and src/sim-cmd.ts so they no longer cite the deleted file. Technical nuance ("env-loading is inlined to avoid triggering config.ts CLI arg parsing when imported from tests") is preserved.

The Vapi /eval REST endpoint, the evals resource type, the /eval/simulation/* simulation API, and docs/learnings/simulations.md are all unaffected — only the local CLI runner is gone.

Customer-fork callers: if any of your automation still invokes npm run eval, swap to npm run sim -- <org> --suite <name> --target <name>. See AGENTS.md for syntax.

Out of scope (pre-existing, not introduced here):

• Pre-existing unresolved git merge-conflict markers in README.md around lines 520-530. Fix separately.
• Stale back-references to src/eval.ts inside improvements.md and requested improvements.md. Those are user-curated trackers and per repo convention are not modified in feature/chore PRs.
• The canonical repo has no biome.json/prettier config — running biome defaults reformats files spaces→tabs. Caught in this PR and reverted; worth a separate hygiene PR to pick a convention and lock it in.

Skipped pipeline stages:

• test-writer skipped — pure deletion of unreachable code with no new behavior to specify. The existing tests/sim.test.ts already covers the surviving runner.
• code-reviewer ran and approved with zero blocking findings.

Test plan

• `npm run build` clean (tsc --noEmit, 0 errors)
• `npm run test` — 115/115 tests pass
• `npm run eval -- foo -s bar` errors with `Missing script: "eval"`
• `npm run sim` (no args) still prints its usage banner
• `grep -rn 'eval.ts' --include='.ts' --include='.json' .` returns zero hits in source/scripts (remaining mentions only in user-curated improvements.md / requested improvements.md, intentionally untouched)