Seal the plan as a specification instead of four bullet lists

The plan artifact was Changes{Target,Intent} plus three string lists, every
entry capped at 500 single-line characters. That bound makes a specification
impossible: a phase cannot carry a code block, a paragraph of reasoning, or a
verification command with its own argument list. renderSealed then flattened
what little survived through collapse(), so an implement session received a
summary of a summary.

plan.md replaces it. Markdown, 128 KiB, no per-line cap, sealed through the
existing path under the existing PlanRef. The parser enforces the structure the
brief states: required sections, phases numbered from 1 with no gaps, Files,
Changes and Verification per phase, and at least one automated or manual check,
because a phase nobody can verify can never be established as done. Automated
entries are JSON argv arrays, so a pipe is a literal argument rather than an
operator. Headings inside fenced blocks are content, so a plan may show
markdown without parsing its own example.

Citations resolve at seal time against the accepted research, on the
coordinator, which is the only party holding ResearchRef. A plan resting on a
finding nobody recorded fails on the planner while its session is still alive
to be told.

The plan now renders byte for byte into the implement launch, and a rotated
successor receives the same complete document. That is the property the whole
change exists for. collapse() stays for research findings, which really are
short claims.

DecodeStoredPlan reads pre-markdown refs and renders them into the same type,
labelled, so nothing downstream branches on which era a plan came from. A
legacy plan carries no phases, which is honest: the old artifact never named an
executable unit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CVbaKucEYBjMqVeUgJUsc1

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