Add pagination to discovery fetches

[JSONbored]

Problem: Discovery fetches request only a single page (max 100 results) with no Link-header follow loop — a repo with over 100 open issues, or a search with over 100 hits, silently truncates.

Area: AMS / Miner

Proposal: Add Link-header-following pagination to both the target-issues fetch and the search-issues fetch in the discovery fanout.

Deliverables:

• Full pagination for both fetch paths, with a sane overall result cap to avoid runaway fetches.

Acceptance criteria:

• A repo/search with more than 100 matching results returns all of them, not just the first page.

Resources:

• The opportunity-fanout module.

[linear-code]

JSO-3491