v0.9.86 — Fix false 'agent switch requires a new session' on new sessions

Fixes

• Stop the false AGENT_SWITCH_REQUIRES_NEW_SESSION 409 that blocked new sessions on the second message. The proxy now treats the default agent sentinel as non-binding and only blocks a genuine switch between two distinct concrete agents; for default sessions the echoed agent is stripped so OpenCode runs its booted default_agent.
• Agent picker: clearer hover tooltip explaining a session's agent is fixed at start (start a new session to switch); switcher behavior unchanged.
• Includes billing duplicate-suppression and Platinum snapshot upload fixes already on main.

What's Changed

• chore(release): VERSION → 0.9.86 [skip ci] by @github-actions[bot] in #3944
• fix(snapshots): stream Platinum build-context upload — fix api-pod OOMKill by @kubet in #3946
• chore(dev-eks): deploy dev-c2217c6e [skip ci] by @github-actions[bot] in #3947
• fix(api): suppress idempotent billing duplicate errors by @agent-kortix in #3921
• chore(dev-eks): deploy dev-3fa2753b [skip ci] by @github-actions[bot] in #3949

Full Changelog: v0.9.85...v0.9.86