feat: accept covers globs in hub schema (inert, lint-validated)

[Connorrmcd6]

Closes #94.

Carves the small, safe, shippable slice out of the covers research epic (#5): the field now exists and is authorable, but it is inert in the verdict — exactly the constraint #94 demands.

What changed

• Schema (surf-core/src/hub.rs): optional covers: Vec<String> on Frontmatter — repo-root-relative globs, same dialect as config.hubs. skip_serializing_if empty, so existing hubs are byte-unaffected on round-trip.
• Tests: replaced covers_field_is_rejected with covers_field_is_accepted + round-trip + default-empty.
• Lint (surf-cli/src/lint.rs): a malformed covers glob blocks (reusing the glob::Pattern::new path --files already uses in check, check --files / stats: invalid glob syntax is silently dropped #38) — silently dropping it would change scope with no signal. As a nicety, a hub whose globs don't match its own anchored files warns (catches fat-fingered globs).
• Docs/dogfood: updated the proposal note (§9.1) that said covers was "deliberately absent", and the hub-format hub claim that asserted unknown fields like covers are rejected — the anchor is on parse_hub, whose span doesn't change, so the hash stayed green while the prose would have silently gone false. Also added a real covers: block to hub-format.md to dogfood the field.

Non-negotiable constraint — held

The verdict never reads covers. surf check is byte-for-byte identical with or without a covers block — verified on this repo (exit 0 both ways, the hub-format claim stays green).

Out of scope (stays in #5)

The advisory coverage pass that diffs changed files against covers globs, and any reviewer-plugin / LLM consumption.

Verification

• cargo test -p surf-core / -p surf-cli — green (new accept/round-trip/default-empty + lint covers tests pass).
• cargo fmt --all + cargo clippy --all-targets -D warnings — clean.
• Dogfood: surf check exit 0 with and without the covers block; surf lint reports 0 errors and no covers findings (valid glob, matches its own anchor).

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