Integrate upstream block/buzz main (35 commits: onboarding refresh, ACP session scope, thread-context dedup)

[mfethe1]

Summary

Integrates upstream block/buzz main (a9dd4d3) into our product/main. We were 243 ahead / 35 behind; 363 files touched on both sides; 15 files conflicted for real.

Resolution strategy

Area	Decision
Pi presets (presets.rs, TESTING.md, URLs)	Take upstream — they moved to their own buzz-pi-acp fork
buzz-acp lib.rs	Keep our SQLite session store + early base_prompt build; drop our pi-launcher (superseded upstream)
pool.rs	Keep our session-store pool; port upstream block#7620 thread-context dedup (hydrated_thread_roots VecDeque, MAX cap, mark_scope_delivery_success 4th param) on top
AgentConfigFields.tsx	Take upstream's restructure; re-apply our #52 openai-compat base-url field (PersonaProviderBaseUrlField + env hint)
pubkey-normalization/truncation/agent-identity files	Keep ours (our gates depend on them)
compose/helm/ARCHITECTURE	Keep ours
mobile/app.dart	Manual merge: overlay never blocks navigator

Verification (local, evidence)

• pnpm run check (desktop): ✅ RC=0 — biome + check:px-text + check:pubkey-truncation + check:pubkey-normalization + check:agent-identity
• cargo test (crates/buzz-acp): ✅ 987+9+9 passed, 0 failed
• cargo test (desktop/src-tauri): ✅ 3247+7+3 passed, 0 failed
• tsc --noEmit (desktop): ✅ RC=0
• Sidecar binaries stubbed per CI convention (_ci-desktop-macos.yml touch) — artifacts only

Key upstream changes absorbed

• Refresh desktop onboarding flow block/buzz#7528/Fix desktop onboarding regressions block/buzz#7659 onboarding refresh (kept ours where ours fixed Relay silently degrades when DB is unreachable; desktop re-presents onboarding for returning users #49/Desktop: "Setting up your welcome team…" spinner never resolves when harness setup was skipped #50/fix(onboarding): profile-check failure never re-onboards a returning user (#49) #63 first — verified fetch-failed re-onboard logic survived)
• Configure ACP session scope per agent block/buzz#7578 per-agent ACP session scope (session_policy on AgentDefinition)
• Deduplicate thread context by ACP session delivery block/buzz#7620 thread-context dedup by ACP session delivery

Closes the sync gap noted in the fleet survey. Follow-up tracked: watch upstream monthly; next sync target ~30 commits/month cadence.