Plan: correct two claims in the Phase 2 reference notes

[EvanWinget]

What changed

Two corrections and one precision fix to the Phase 2 reference block that PR #15 added to docs/execution-plan.md, from a post-merge review Evan requested. No spec, implementation, or vector changes, and no skeleton element touched.

1. Message-condition pricing precedent. The block said Chia's announcement pricing maps onto our SEND/RECV_MESSAGE conditions as their closest descendant. Verified against CHIP-0025 and CHIP-0049 (cited by name deliberately, no links): our messages port Chia's SEND_MESSAGE/RECEIVE_MESSAGE, the announcements' successors, and CHIP-0049's always-priced exception enumerates only the four announcement codes, so Chia's own message conditions apparently ride the general free tier. The precedent is split, and the note now says so instead of implying the announcement treatment carries over. The free tier's per-coin-spend granularity is now stated too, since the exemption multiplying across spends is the DoS-relevant part.
2. Obligation 2 gloss. Obligation 2 commits to per-condition costing tuned against UTXO-set growth with superlinear CREATE_COIN pricing. It does not state a pricing-over-caps philosophy, so "commits us to" becomes "consistent with obligation 2's pricing approach".
3. Decided vs open. The taproot bullet's headline asserted the condition-layer answer while deferring the question. It now reads "out of the VM (ratified), the condition-layer form open", matching the D2 record's actual scope, and "answer" becomes "candidate" in the body.

How to verify

• git diff main~1 -- docs/execution-plan.md shows the original block plus this correction as one readable unit.
• Obligation 2's text: docs/bitcoin-script-successor-evaluation.md section 7.
• The D2 muladd decline: spec/VM.md section 8, D2 entry.
• PATH=.venv/bin:$PATH ci/lint/lint.sh