chore(plans): drop shipped/absorbed plan docs ahead of v0.10.0

Removed plans whose work landed and whose durable learnings already live in
the skill references:
- obol-sell-demo.md — implemented (`obol sell demo` ships in the CLI)
- post-490-integration-20260513.md — integration landed in May
- release-smoke-hardening-20260513.md — session retro; learnings folded into
  obol-stack-dev/references/release-smoke-debugging.md
- inference-v1337-{buy-report,followup}-20260514.md — QA reports; the WAF/UA
  findings live in release-smoke-debugging.md §10 (c2dddc1)

Kept: sell-agent-perf.md (in progress), openapi-402-followups.md (pending
walk-through), openapi-redoc-storefront.md (phase 2 deferred),
storefront-buy-inference-cta-handoff.md (active frontend handoff),
volume-permission-hardening.md (PR #615 owns its rewrite).

Author: bussyjd