docs(guides): DASHBOARD_ALLOW_EMBED is a build-time flag, not a runtime one

[diegosouzapw]

Problem

docs/guides/VSCODE-COPILOT.md told operators to start OmniRoute with
DASHBOARD_ALLOW_EMBED=vscode omniroute. That never works on a prebuilt install.

Next.js compiles headers() into the route manifest, so next.config.mjs reads the variable
while the bundle is built — the comment at next.config.mjs L83-86 says so explicitly
("Read at build time … so changing it requires a rebuild"), and
docs/reference/ENVIRONMENT.md + .env.example already documented it correctly. Only the
how-to guide was wrong, and it is the page a user actually follows.

Consequence: anyone on npm install -g omniroute or the Docker image sets the variable,
restarts, and the dashboard still refuses to frame — with no hint why. The OmniCopilot
extension's own fallback message already words it right ("a build-time option, so setting the
variable on an existing install is not enough"); the guide contradicted it.

Change

Docs only — no source touched.

• Replaces the runtime recipe with the build-time one (DASHBOARD_ALLOW_EMBED=vscode npm run build)
• Adds a table of which install paths can enable embedding at all (source ✅, global npm ❌, Docker ❌)
• Fixes the troubleshooting row that repeated the same wrong instruction

Known gap (not fixed here)

The Dockerfile has no ARG DASHBOARD_ALLOW_EMBED, so Docker users cannot pass it through
--build-arg — they have to edit the Dockerfile. Adding the ARG+ENV pair next to
OMNIROUTE_BASE_PATH (L140-141) would turn that ❌ into a ✅; deliberately left out of a
docs-only PR.

Verification

npm run check:docs-all — check-doc-links PASS (841 internal links), check-fabricated-docs
PASS. The drift warnings it prints are pre-existing and advisory.

⚠️ base-red inherited: #9985

Refs #10273

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The Merge integrity red on this PR is inherited, not caused here: check:agent-skills-sync fails identically on a pristine checkout of the base tip. Discriminated and fixed in #10698.