refactor(buck2): make repository checks standalone

[schickling-assistant]

Problem

Repository aggregate checks still carried composition-era graph edges after effect-utils adopted a tracked standalone Buck root.

Goal

Make the repository check surface standalone, remove mr prerequisites from the check graph, and refresh the recorded graph evidence without changing the remaining explicit cross-repository composition contract.

Decisions

• Remove mr:setup, mr:check, mr:lock-sync-check, and mr:source-policy-check from the standalone check graph.
• Keep canonical effect_utils// platform and target labels.
• Preserve standalone bootstrap wording and the bootstrap → Genie freshness dependency inherited from feat(buck2): add standalone root and aggregate gates #1301.
• Treat cross-shape action-key comparison as migration evidence, not a permanent parity requirement.
• Keep exactly two own commits: implementation followed by refreshed evidence.

Verification

• Standalone task-graph proof: all 205 assertions passed.
• The proof confirms the removed mr tasks are absent and check:all has no mr:apply dependency.
• Required GitHub checks on the latest revision are the merge gate.

Complexity

• Ledger: 43 lines added, 23 removed, net +20, measured by buck2-ledger.ts at 90a99162aeaa0178339918a1a95b97828dfb57ba.

Concerns

• This PR removes effect-utils check-graph composition edges only. It does not delete the shared mr machinery still needed by downstream repositories.

Friction & bottlenecks

• Rebase conflicts were limited to overlapping task-graph assertions. The stricter inherited guards remain: ordinary tasks do not reach mr:apply, and editor bootstrap reaches neither mr:setup nor genie:check.
• /srv/bulk Buck timings are invalid under dotfiles#2955; the feat(buck2): add standalone root and aggregate gates #1301 local-disk control run proves the inherited standalone root and pinned Buck path.
• The q66 generator-closure fix and DELTA-002 are folded into feat(buck2): add standalone root and aggregate gates #1301. Run 35761554038 exercises this head.

Follow-ups

• Delete the paused composed shape at L3 cut 2 after remaining downstream edges move.

References

• Depends on feat(buck2): move bounded Weaver checks #1302 and feat(buck2): add standalone root and aggregate gates #1301.
• Stack: #1332.

Posted on behalf of @schickling

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