fix(digest): cap followups-due section by --limit

[Yurii214]

What changed

vouch digest caps its pending, decisions, and stale sections to --limit, but the followups-due section was built without the slice, so it returned every due followup regardless of the limit. this adds the same [:limit] slice the sibling sections use.

Why

the --limit help reads "cap per section (pending, decisions, stale, followups)" — followups is documented as capped, but the code didn't honor it. vouch digest --limit 10 against a kb with many due followups returned the full list in that one section while every other section was bounded.

What might break

nothing on disk. digest is a read-only viewport: no file moves, no field-shape changes, no kb.* behavior change. the only observable change is that followups_due (and the text/markdown renders) now show at most limit rows, like the other three sections.

VEP

not a surface change; none needed.

Tests

• make check passes locally (ruff + mypy + pytest: 1195 passed, 34 skipped)
• new behaviour has a test: test_build_limit_caps_followups — fails on the previous code, passes now
• CHANGELOG.md updated under ## [Unreleased]

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