docs: state that build output is never a lint/format/typecheck target

Adds an AGENTS.md rule under the pre-push gate naming the gated surface
(first-party source) and everything excluded from it (each client's build/,
web's dist/ and storybook-static/, coverage/, test-servers/build,
core/**/{build,dist}, *.tsbuildinfo), why gating generated output is harmful
even when it only warns, and the fact that the three coverage guards assert
the converse and so cannot catch this drift. Mirrored into
.github/copilot-instructions.md per the maintenance rules.

Signed-off-by: cliffhall <cliff@futurescale.com>

AGENTS.md

@@ -748,6 +748,19 @@ The ⚠️ option-deletion hazard, the snapshot rule, and the recovery recipe ab
 - `smoke:cli` (`scripts/smoke-cli.mjs`) drives `mcp-inspector --cli` through the built launcher against the bundled stdio test server via a temp `--catalog`: it asserts `tools/list` returns the server's tools (real connect over stdio), the default writable catalog is seeded empty on first run, a missing read-only `--config` errors without seeding, and `--catalog` + `--config` is rejected. `smoke:tui` (`scripts/smoke-tui.mjs`) launches `mcp-inspector --tui --catalog <temp>` and asserts the Ink app renders its first frame (the "MCP Servers" panel) within a timeout, then SIGTERMs it — a shallow boot/render check, not full interaction. **`smoke:tui` is local-only: it self-skips when `process.env.CI` is set**, because the Ink TUI needs a real TTY (raw mode) that headless CI lacks — so run it (via `npm run smoke`) on your own machine before pushing. Both build `test-servers/build` on demand if it's missing.
 - Storybook play-function tests (`clients/web` `test:storybook`) run in headless Chromium via `@vitest/browser-playwright` (~10s). They are part of `npm run ci` (which installs Playwright chromium first); kept out of `validate` because they need the browser binary and are slower than the unit suite.
 
+### Build output is never a gate target
+
+**No gate — lint, format, or typecheck — may read generated output.** The gated surface is first-party source only: `clients/*/src`, `clients/*/__tests__`, `clients/web/{server,.storybook}`, each client's top-level configs, `core/`, `test-servers/src`, `scripts/`, and the root shared files. Everything a build writes is out of scope: `clients/*/build` (the tsup/tsc bundles), `clients/web/dist` (the Vite SPA), `clients/web/storybook-static`, `clients/*/coverage`, `test-servers/build`, `core/**/{build,dist}`, and any `*.tsbuildinfo`. Each scope states this itself — `globalIgnores([...])` in every `eslint.config.js`, the `format`/`format:check` globs in each `package.json`, and a tsconfig `include` that names source directories rather than the package root.
+
+Why it matters, given that these paths are all gitignored and the findings are usually warnings:
+
+- **It reports defects nobody can fix.** A bundle vendors third-party code, so a rule that fires inside it names a problem in someone else's source. `clients/web` shipped this for a while: `build` was missing from its `globalIgnores` while its three sibling clients had it, so the client's *only* lint output was an unused-`eslint-disable` warning from inside the vendored `undici` (#2043).
+- **A warning becomes a gate failure without warning.** `reportUnusedDisableDirectives` is a warning by default and a rule promotion — or any new rule a bundled dependency happens to trip — turns it into a `validate` failure on a file nobody wrote. #1959 (enabling `no-floating-promises` across all five scopes) is exactly that kind of change.
+- **It trains people to skim the channel.** A scope whose lint is never clean has no signal left in it, and the real warning added later lands where everyone has learned to look past.
+- **It is wasted work on every run.** `lint` runs inside `validate`, the fast inner-loop check, and the web runner bundle alone is ~1.2MB of generated JS.
+
+The three coverage guards do **not** catch this, and adding a fourth is not the fix. `verify:format-coverage` and `verify:typecheck-coverage` assert that first-party source is *covered*; neither asserts that generated output is *excluded* — an asymmetry that is deliberate, since a guard can't distinguish "generated" from "source" without being told, and the ignore lists are already that statement. So this class drifts silently and the check is a human one: **when a build starts writing to a new location, add it to that scope's ignore list in the same change.** The reverse of the guards' rule also holds — never widen an ignore to silence a finding in first-party code, and never add a build directory to a tsconfig `include` to make a generated `.d.ts` resolve (import the source, or fix the build's types).
+
 ### Typescript instructions
 
 - Use TypeScript for all new code