fix(lint): scope Biome off pre-existing web-client and docs debt

[chopmob-cloud]

What this does

The required Lint Code Base check runs Super-Linter's Biome across the whole repository, so every PR's Lint run reports the pre-existing Biome errors in code/web-client and docs/assets (75 diagnostics in recent runs) rather than only the PR's own changes. This is the out-of-diff scanning described in #306, and it currently reds otherwise-clean PRs.

This adds a minimal biome.json that excludes those two directories from Biome:

{
  "files": {
    "includes": ["**", "!code/web-client", "!docs/assets"]
  }
}

Tradeoff (please decide deliberately)

This is a coverage exclude, not diff-scoping. It removes code/web-client and docs/assets from Biome for all future changes in those directories, not only for the current diff, for as long as this configuration applies. It unblocks the required gate immediately and touches no workflow (so the zizmor gate is unaffected) and no source, but it trades away Biome coverage of those two trees.

The alternative is #311, which instead clears the existing Biome errors so Biome keeps full coverage. The two approaches target the same goal and are mutually exclusive:

• this PR: exclude the debt directories now, unblock the gate, accept the coverage cost;
• fix(lint): clear the pre-existing Biome errors so the lint gate reflects PR content #311: fix the debt, keep coverage, larger change surface.

Posting this as the minimal-unblock option so maintainers can choose explicitly between the two. If #311 is preferred, this PR can be closed.

Type of Change

• Maintenance (CI)