feat: admin feed list/create/update/sync

[nsollazzo]

Summary

• Admin positronick feed commands wrapping /api/admin/feeds (list / create / update / sync)
• Mock handlers + API client tests + CLI goldens
• Enables release subscriptions without raw curl

Golden / contract

• New goldens: feed-list.json / feed-list.txt, feed-create.json / feed-create.txt
• agent-docs gains feed sections for cached admins
• Table columns: ID LABEL KIND LISTING AUTHOR AUTO ON LAST URL
• JSON: list {count,feeds}, create {feed,created}, update {feed}, sync {summary}

Test plan

• go test ./... -race
• Local binary: feed list against production (admin session)
• Idempotent verify Buzz + Grok Build feeds (already present; sync OK)
• OpenCode feed created via new CLI when listing existed

Ops note

Production already had Buzz listing + Buzz/Grok feeds from prior admin API; this PR is the durable CLI path. OpenCode release feed was added with the new binary after list showed it missing.

[nsollazzo]

Superseded by work already on main:

• feat: hidden positronick feed admin commands (list/create/update/sync) #27 feat: hidden positronick feed admin commands (list/create/update/sync) (68e22d3)
• later refactors (refactor: dedup admin field-flag application into applyStringFieldFlags (POS-187) #36 field-flag dedup, Go 1.26.5, posts/research, …)

This branch was started from an older tip and conflicts as add/add on feed.go. Closing rather than force-merging a parallel surface. Ops will install/build from current main and smoke-test positronick feed against production.