build: lower the Node engines floor to 22.18

docs/rfc/implemented/process/2026-06-11-quality-gates.md

@@ -14,7 +14,7 @@ Every AGENTS.md promise gets a command that exits non-zero, wired into git hooks
 - ESLint strict-type-checked + @stylistic (the house style, enforced); vendored code excluded.
 - Per-file 100% coverage on `packages/*/src` (v8); unreachable defensive guards carry `/* v8 ignore */ ` with stated reasons instead of deletion.
 - knip (dead code/deps), publint (package correctness), workspace constraints (workspace rules: private, cordis peer+dev, uniform version, ESM), and a NodeNext consumer typecheck for built package declarations.
-- lefthook pre-commit (lint staged, typecheck, vendor-manifest guard) and pre-push (tests, hygiene); CI runs the full matrix on node 24/26 plus a demo smoke test driving the echo-agent end to end.
+- lefthook pre-commit (lint staged, typecheck, vendor-manifest guard) and pre-push (tests, hygiene); CI runs the full matrix on node 22.18/24/26 plus a demo smoke test driving the echo-agent end to end.
 
 ## Consequences
 

docs/rfc/implemented/process/2026-07-06-node-22-18-floor.md

@@ -0,0 +1,30 @@
+# RFC: Lower the Node engines floor to 22.18
+
+Status: implemented
+
+## Problem
+
+The root `engines.node` was `>=24`, which excluded the entire Node 22 LTS line for no runtime reason. The harness has exactly two Node features whose availability gates the floor, and both are satisfied well below Node 24 — so the floor was higher than the code actually requires. Pinning it honestly widens the supported install base (Node 22 LTS is in service until 2027) without weakening any guarantee, provided CI proves the claim on the floor version rather than merely asserting it in a manifest.
+
+## Decision
+
+Set `engines.node` to `>=22.18` and treat 22.18 as the tested floor everywhere (CI matrix `['22.18', 24, 26]`, the real-API e2e job on `['22.18', 24]` — floor plus primary line, since the keyless matrix exercises only the mock adapter and the live `fetch`/SSE path runs only in e2e). 22.18 is the *later* of the two feature boundaries the code depends on, so it is the earliest Node version where everything the repo ships and tests runs unflagged:
+
+- **`node:sqlite` — Node 22.13.** `packages/session-persistence/session-persistence-sqlite` does a top-level `import { DatabaseSync } from 'node:sqlite'`. The module dropped its `--experimental-sqlite` flag requirement in Node 22.13 (backport of the 23.4 change), so any floor ≥ 22.13 loads it without a flag.
+- **Native TypeScript type-stripping — Node 22.18.** The `packages/ui/stdio-agent/tests/built-bin.e2e.ts` smoke boots the published `lib/bin.js` under plain `node` and loads the example's `.ts` plugins (`mock-llm.ts`, `echo-tool.ts`) with no tsx. Native type-stripping — which makes that work — was unflagged in the 22.x LTS line only in 22.18 (before that it needed `--experimental-strip-types`). This is the binding constraint, so it sets the floor.
+
+`@types/node` is pinned to the 22.x line (`^22.20.0`) to match the floor: reaching for a Node 23+/24+/25+ API then fails `tsc` on every machine and in the typecheck gate, rather than compiling clean and surviving to a runtime failure only the 22.18 matrix leg could catch. The whole tree typechecks clean against the Node 22 type surface today, so the pin costs nothing.
+
+## Consequences
+
+- The supported base widens to the Node 22 LTS line, and the `['22.18', 24, 26]` matrix proves it on every push and PR rather than trusting the manifest.
+- The built-bin smoke needs no version-conditional flag: at 22.18 type-stripping is already the default, so the test stays the plain `node lib/bin.js` path it documents.
+- A future change reaching for a Node 23+ API fails `tsc` immediately (the `@types/node` pin); one reaching for an API added in 22.19/22.20 — inside the 22.x type surface but above the floor — is caught instead by the 22.18 matrix leg. Either way the floor must move in the same change.
+- The `vendor/hmr` and `vendor/loader` comments about Node 24 module-cache internals are unaffected — they describe dev-time HMR loader behavior, not the shipped runtime contract, and are pinned vendored source.
+
+## Alternatives considered
+
+- **Floor `>=22.13` (the `node:sqlite` boundary) plus `--experimental-strip-types` in the built-bin smoke on 22.13–22.17.** Rejected: it adds a version-conditional test flag for one narrow range and dresses up an experimental-flag dependency as first-class support. 22.18 clears both boundaries with zero test special-casing, and the five-patch gap below it buys nothing real.
+- **Keep `>=24`.** Rejected: it excludes Node 22 LTS with no runtime justification once the two boundaries above are known.
+- **Matrix `[22, 24, 26]` (latest 22.x) instead of pinning `22.18`.** Rejected: "latest 22.x" drifts upward over time and would silently stop exercising the declared floor. Pinning the floor version is what makes the matrix a proof of the claim rather than a proof of some newer 22.x.
+- **Keep `@types/node` ahead of the floor (`^25`).** Rejected: types ahead of the runtime floor let a Node 24/25-only API compile clean and fail only at runtime on 22.18 — exactly the "green types, broken product" gap. Pinning `@types/node` to the 22.x line turns that into a compile error everywhere, and the tree already typechecks clean against the Node 22 surface, so the pin is free.