docs: record rejected alternatives in DECISIONS.md

Adds a consolidated table of approaches considered and deliberately not taken
(Python, Supabase, living in the monolith, shared-lib lift, filesystem brain
package, reusing inbound oauth, global dedup table, Whisper-in-core, SaaS-now,
swarm-delegating the spike), each mapped to the ADR that settles it. Prevents a
later session from re-proposing settled rejections as fresh ideas.

DECISIONS.md

@@ -166,3 +166,27 @@ the repo simultaneously. CI is the quality gate, not branch protection.
 
 **Consequences.** Consistent with the rest of the estate. Depends on the direct-to-main write
 path tracked in gitea-mcp #35 (item #1).
+
+---
+
+## Rejected alternatives
+
+Approaches considered during the 2026-06-02 planning + grill session and **deliberately not
+taken**. Recorded so a later session doesn't re-propose them as if they were fresh ideas. Each
+maps to the ADR that settles it.
+
+| Rejected | Why rejected | Settled by |
+|----------|-------------|-----------|
+| Python / FastAPI / arq stack | Would be the only non-Go service in the estate, outside every shared chassis/convention/CI pattern | ADR-001 |
+| Self-hosted Supabase (auth + RLS + Vault) | Duplicates Dex, ESO+1Password, and the postgres instance being actively de-coupled; three new things to run and back up | ADR-002 |
+| Living inside the `hyperguild/ingestion` monolith | The only worthwhile reuse is ~100 lines (`llm`) that get copied anyway; standalone-first wants the service to owe the monolith nothing | ADR-003, ADR-004 |
+| Lifting shared packages into a `brain-common` module | Couples Tapir's release cycle to the monolith for negligible code savings | ADR-004 |
+| Importing/replicating the filesystem `brain` package | Assumes co-location with the brain git checkout; wrong for a standalone networked service | ADR-005 |
+| Reusing `ingestion`'s `oauth` package for YouTube/Vimeo | Same name, opposite direction — it's inbound MCP-server auth, not outbound provider OAuth | ADR-006 |
+| Global cross-tenant `videos`/`transcripts` table (dedup) | Reintroduces the cross-domain DB coupling the homelab review is removing; at 1–5 users, re-summarizing is cheaper than the coupling | data-model.md |
+| Audio-download + Whisper STT in the core path | ToS-grey, breakage-prone (yt-dlp), contends for koala GPU with the JEPA PoC; captions alone test the core hypothesis | ADR-007 |
+| Building multi-tenant SaaS / Google OAuth verification now | "Real users soon" was lowered to Future B; SaaS machinery before the Stage 0 self-use gate is the primary documented anti-goal | ADR-008, VISION |
+| Delegating the S5 reuse spike to an agent swarm | A 1-hour sequential read-and-judge with a single coupled conclusion; orchestration overhead exceeds the work, and it's Diamond-1 judgment the maintainer wanted to own | (process note) |
+
+If a future case genuinely reopens one of these, that's a new ADR superseding the relevant one —
+not a silent reversal.