fix(build): stop --dry-run deleting dist/ and honor build.outDir

[kojiwakayama]

Found on a DX dogfood walk: a fresh project was scaffolded, veryfront build
was run against it from outside the monorepo, and the output directory was
watched.

Symptom

Two separate ways for veryfront build to destroy a project's dist/ with no
warning.

1. --dry-run deletes the output directory. With three files in dist/
(index.js, nested/deep.txt, a marker file):

$ veryfront build --dry-run
  ! Dry run: no files will be written
  ✓ Built in 0.65s
    1 page, 0 chunks, 0 assets
    4.63 KB in dist
$ find dist
find: dist: No such file or directory

The directory and everything in it is gone, and nothing was written to replace
it. A developer who runs a dry run to preview a build loses their previous
build output.

2. build.outDir is silently ignored. With
export default { build: { outDir: "custom-out" } } in veryfront.config.ts,
the build still cleared and wrote dist/ and reported 4.63 KB in dist;
custom-out/ was never created. No warning, no error. The config file is
loaded — the same file with ssg: false changes the build's behaviour — so
outDir was read into the config object and then dropped. That left -o/--output
as the only working escape hatch, and nothing in the CLI says so.

Root cause

setupBuildDirectories (src/build/production-build/build/build-setup.ts) ran
an unconditional adapter.fs.remove(outputDir, { recursive: true }) before
its if (dryRun) return guard, so a dry run cleaned the output directory and
then skipped everything that would have refilled it.

buildCommand (cli/commands/build/command.ts) hardcoded
options.outputDir ?? join(options.projectDir, "dist"), computed before
getConfig() was even called, so config.build.outDir never reached the build.

Fix

• Move the clean step behind the dry-run guard. A real build still clears its
  output directory, so stale artifacts from a previous build cannot leak into a
  new one.
• Resolve the output directory from the loaded config:
  -o/--output > build.outDir > dist. A relative outDir resolves against
  the project directory. displayBuildConfig moved after getConfig() so the
  displayed, reported and written directory are the same one.
• build.outDir moves from "accepted for compatibility, no built-in semantics"
  to "core runtime/build" in src/config/README.md, and docs/guides/deploying.md
  no longer tells readers the builder ignores it.

Regression tests

Test	Why there
src/build/production-build/build/build-setup.test.ts — "leaves an existing output directory untouched in dry run" / "still clears the output directory for a real build"	Unit-level, next to the guard that was in the wrong order. The pre-existing tests in this file use a mock adapter whose remove is a no-op, which is precisely why they could not see the deletion; these use an adapter that deletes for real.
cli/commands/build/command.test.ts — resolveBuildOutputDir cases	Pure resolution rules (flag beats config, config beats default, absolute stays absolute), alongside the existing formatBuildOutputPath tests.
tests/integration/server/build/build.test.ts — new "output directory" block	End-to-end through the real config loader and the real build. The build.outDir case drives buildCommand itself, since that is where the directory is decided, and asserts on what the CLI prints (… in custom-out) — the exact line that read … in dist before. Added inside the existing suite rather than a new file so no new sanitizer opt-out is introduced (lint:sanitizer-baseline stays at 404/404).

Both integration tests were confirmed red before the fix: the dry-run one fails
with NotFound … /dist/index.js, the outDir one fails with the captured CLI
output showing 4.63 KB in dist.

Also verified against the real CLI on a scratch project outside the repo:
--dry-run now writes and deletes nothing; build.outDir: "custom-out" writes
to custom-out/ and leaves dist/ intact; -o flagout still overrides the
config; with no config the default is still dist and stale artifacts are still
cleared.

Review follow-up

Honoring build.outDir made a previously inert config value load-bearing, and
review caught the one new hazard that created: since the build clears its output
directory before writing, an outDir of . or .. would have recursively
deleted the project's own source, or the workspace above it. A compatibility-era
config that was harmless while the field was ignored would have become
destructive.

resolveBuildOutputDir now rejects any output directory that is the project
directory or contains it, and throws rather than quietly falling back to dist —
silently substituting a different directory is the ignored-configuration bug this
PR exists to fix. The guard covers -o/--output as well as the config path;
veryfront build -o . could already delete the project before this PR, and a
guard on only one of the two inputs to the same function is a half-guard. A
sibling directory such as ../shared-dist is unaffected.

Also added the missing import { defineConfig } from "veryfront"; to the
deploying.md snippet, matching every other config example under docs/guides/.
On a PR whose whole premise is following the published docs literally, a snippet
that does not compile when pasted is in scope.

Declined on the record: --preset embedded still ignores build.outDir. It also
ignores --dry-run (it writes files anyway), --no-split, --no-compress,
--prefetch, --ssg/--no-ssg, --include and --exclude, and never calls
getConfig() at all — handleBuildCommand branches to handleEmbeddedBuild
before the config is loaded and passes it exactly one option. That is a
pre-existing, uniform condition this PR neither created nor worsened, and fixing
outDir alone would leave the same silent-ignore for six other flags. Filed as
#3585.

Not fixed here

A build that fails still leaves the developer with neither the old artifact
nor a new one, because the clean happens up front. Fixing that properly means
staging the build and swapping it in at the end — createBuildPublication in
src/build/production-build/build/build-publication.ts already does exactly
this for the asset pipeline and is the obvious vehicle — but routing the whole
production build through it is a much larger change than this one and belongs
in its own PR.

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📥 Commits

Reviewing files that changed from the base of the PR and between 718355c and e3b8c68.

📒 Files selected for processing (8)

• cli/commands/build/command-help.ts
• cli/commands/build/command.test.ts
• cli/commands/build/command.ts
• docs/guides/deploying.md
• src/build/production-build/build/build-setup.test.ts
• src/build/production-build/build/build-setup.ts
• src/config/README.md
• tests/integration/server/build/build.test.ts

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