fix(type-safety): replace manual typeguards with valibot in SPA & reddit-fetch

[la14-1]

Summary

• Replaces all as Record<string, unknown> casts and manual multi-level typeguard chains with valibot schema validation in SPA main.ts and reddit-fetch.ts
• Adds RedditTokenSchema, RedditListingSchema, RedditChildDataSchema, and RedditCommentDataSchema with v.safeParse() for all external Reddit API data
• Eliminates 15 type-safety violations (6 in main.ts, 9 in reddit-fetch.ts) flagged in security: Manual typeguards used instead of valibot validation in SPA code #3200

Files changed

• .claude/skills/setup-spa/main.ts — Reddit token auth, error message parsing, jQuery-style comment URL extraction
• .claude/skills/setup-agent-team/reddit-fetch.ts — Reddit auth, listing/post extraction, user comment fetching

Test plan

• Verify bun build passes for both files (confirmed locally)
• Verify no as casts remain in changed files (confirmed: grep returns zero matches)
• Trigger a growth cycle to verify Reddit API parsing still works end-to-end
• Verify SPA Reddit reply posting still extracts comment URLs correctly

Closes #3200

Filed from Slack by SPA

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