Refresh STATE + ROADMAP + concept §3.5 to reflect plugin landscape

[lx-0]

Summary

Three loose ends from PR #1 -- which intentionally deferred concept.md and left STATE/ROADMAP for a follow-up.

What's in this PR

.ytstack/STATE.md:

• Append "Session 2026-04-25 (afternoon)" block covering the plugin-landscape session: 5-plugin family locked, 3 new DECISIONS entries (already merged via PR Lock plugin landscape architecture; ystacks as primary install path #1), Yesterday-AI/ystacks + Yesterday-AI/yastack repos shipped (both with own minimal .ytstack/ for dogfooding), Levels-of-AGI 3-4 framing fix, M010 + M011 surfaced as next substantive work
• Refresh next-action queue with concrete commands inline (no chat-verweise)
• last_updated bumped + current_milestone annotated

.ytstack/ROADMAP.md:

• Add M010 (Workflow Reorder + Brownfield-Without-.ytstack) and M011 (Post-Summarize Lifecycle) as planned milestones with goal + exit criteria + scope-reference to relevant DECISIONS entries
• M010 has two-part scope: finish the original 2026-04-24 greenfield-flow reorder AND add the new "user runs ytstack inside an existing repo with no .ytstack/ yet" case as a third workflow
• M011 wraps vendor/gstack/ship/SKILL.md plus one post-ship skill (canary OR document-release, choice deferred to milestone planning); other ship-family skills are explicitly out of scope per DECISIONS 2026-04-25 lifecycle-heuristic
• Frontmatter total_milestones bumped 9 -> 11

docs/concept.md §3.5:

• Rewritten to reflect ystacks consolidation. Was deferred per PR Lock plugin landscape architecture; ystacks as primary install path #1's marketplace DECISIONS entry "How to apply" line ("docs/concept.md §3.5 update is deferred to a separate change so this DECISIONS entry lands first as the source-of-truth")
• Now describes: monorepo + catalog hybrid; 5-plugin family with locations + visibility; service-repo plugin pattern; ytstack's legacy self-marketplace status
• Concept paper is now consistent with DECISIONS 2026-04-25

Why

PR #1 landed the architectural decisions but intentionally left state-tracking and concept-paper consistency for a follow-up. This PR closes those loose ends so future sessions read a coherent state across DECISIONS / STATE / ROADMAP / concept.md.

Reviewer notes

• No code changes -- documentation + state-tracking only.
• M010 and M011 are entered as planned, not in-progress. Actual milestone planning happens via ytstack:plan-milestone when the user starts each.
• ystacks + yastack repos are not in this repo's scope -- they live in their own repos and have their own .ytstack/ dogfood scaffolds (committed in their respective repos this session).

Test plan

• Read STATE.md "Session 2026-04-25 (afternoon)" block and confirm it accurately summarizes the architecture session
• Read ROADMAP.md M010 + M011 entries; confirm goals + exit criteria + scope-references are coherent with the DECISIONS they cite
• Read concept.md §3.5; confirm it now matches DECISIONS 2026-04-25 "Marketplace consolidates on Yesterday-AI/ystacks (monorepo + catalog hybrid)"
• Confirm no em-dashes ($'\xe2\x80\x94') were introduced
• Confirm no chat-verweise in STATE.md (no "see chat" / "provided in chat" phrasing)

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