feat: add loopx-community-discussion public source provider

[huangruiteng]

Adds an independently packaged LoopX extension (packages/loopx-community-discussion) that turns public community discussion into typed discussion_fact_v0 facts and a community_discussion_scan_v0 document.

Changed surfaces

• New optional extension package: GitHub issues + GitHub discussions (GraphQL, needs GH_TOKEN) and exact-match HN Algolia (typo tolerance disabled to avoid Loopxo/Looptap/CrowdStrike noise).
• Deterministic markdown digest renderer and versioned JSON schemas (fact / scan / request / response).
• Offline contract smoke (smoke/community_discussion_smoke.py) plus optional live scan.
• One row in the value-connectors connector profile table.

Boundary

• Metadata-only facts (title, URL, author, timestamp, relevance, dedupe key); no raw payloads, post bodies, credentials, local paths, or private context.
• No external writes; digest delivery (e.g. Lark) stays an operator-owned, exactly gated step.
• X and Reddit are documented as non-bundled providers (X needs a logged-in browser/XMCP; Reddit public JSON is currently walled/rate-limited).

Validation

• python3 -m py_compile on the new package: passed.
• python3 packages/loopx-community-discussion/smoke/community_discussion_smoke.py: passed (dedupe, schema, noise filter, markdown render).
• Live scan huangruiteng/loopx --days 14 with GH_TOKEN: 65 facts, 0 source errors.
• loopx canary premerge --from-git-diff --goal-id loopx-meta: passed 18/18, change-quality receipt cqr_40a75c4512fd885373ac valid for the exact scope fingerprint 40a75c4512fd885373acf32f3bf15a9829fb547612f5f79ca48839f2ba1dd47e.