Exclude vendored/generated files from the whole check sweep, not just warning-suppression

[missingbulb]

Problem

Only warning-suppression consults ctx.vendored() (files git marks linguist-vendored / linguist-generated) to skip third-party/machine-written content. Every other check still scans those files as if they were the project's own authored code — so a check that fires on a recorded fixture or a generated artifact has no honest escape but a per-rule accept with a reason, which reads as "we accept this violation" rather than the truth: "this isn't our code, don't look at it."

This surfaced downstream: a GoogleCalendarEventCreator session (PR #664) added a subtree accept to silence warning-suppression findings inside cached third-party fixture HTML — clunky config to suppress findings that the vendored-skip (#188) already handles for that one check. The general fix is to make "not the project's code" a property of the sweep, honored by all checks, not a per-check opt-in.

Change

Promote the vendored/generated skip from a single check into the engine:

• checks/lib/context.mjs: compute the linguist-vendored / linguist-generated set once and make ctx.files (the default iteration surface) exclude them. Expose the raw set as ctx.allFiles for the one check that legitimately reasons about generated files.
• packs/universal/warning-suppression.mjs: drop its now-redundant manual ctx.vendored() skip.
• packs/universal/generated-merge-driver.mjs: read ctx.allFiles so it still inspects GENERATED-named files even when a repo also marks them linguist-generated (preserves current behavior exactly).

Link targets (reference-integrity) resolve via ctx.exists() against the real tree, so linking into a vendored file is unaffected; only a vendored file's own outbound links / placement / markers stop being policed, which is correct.

Docs (checks/README.md, context.mjs comment) and tests updated.