chore: integrate LavaMoat/harden

[Mrtenz]

Explanation

Scripts run through yarn run currently execute with the full ambient authority of the developer's shell: unrestricted filesystem access, network access, the ability to spawn child processes, and access to every environment variable including credentials. A compromised dependency that gets to run any lifecycle or package script therefore has a straightforward path to exfiltrating secrets or tampering with files outside the repository.

This PR integrates the LavaMoat/harden setup, ported from MetaMask/metamask-module-template.

It adds a lavamoat/ directory containing:

• Script permission configs with secure defaults — scripts.strict.json (deny all powerful IO capabilities), scripts.loose.json (a more permissive variant for scripts that genuinely need native capabilities), and scripts.skills.json (used by skills:postinstall).
• .env.ban.json — a list of keywords used to filter sensitive environment variables out of the script environment.
• .runner-plugin.js — a Yarn 4 plugin that hooks wrapScriptExecution and applies the above. For every script it resolves the matching config from the workspace's scriptsConfig field, translates it into Node's permission model flags via NODE_OPTIONS, strips banned environment variables, and moves node_modules/.bin to the end of PATH to eliminate bin confusion attacks.
• plugin-allow-scripts.js — the LavaMoat allow-scripts plugin, now vendored locally rather than fetched from a remote URL at install time.

Every workspace declares scriptsConfig.#default pointing at the strict config, and a new Yarn constraint in yarn.config.cjs enforces that this stays in place for newly added packages.

Some changes whose purpose may not be obvious:

• .nvmrc is pinned to Node 26 — the permission model flags used here require a recent runtime.
• .npmrc and .yarnrc are added — these make accidental use of npm or Yarn 1 safe (ignore-scripts, offline), since neither honours the Yarn 4 plugin.
• enableHardenedMode: true in .yarnrc.yml validates lockfile content against the remote registries.
• enableMirror: false and globalFolder: .yarn/global — despite enableGlobalCache being disabled, Yarn still writes to the global folder, so it is redirected into the project to stay within the permitted write scope.
• lavamoat/ is excluded from ESLint and oxfmt — the plugin sources are vendored from upstream and are intentionally kept unmodified so they remain easy to diff against the template.

References

Based on MetaMask/metamask-module-template#335

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them