Lane B: decompose deltas — prefactor-first, window sizing, integration-branch fallback, frontier phrasing, PR-variant briefs

[kyle-sexton]

Parent

Refs #2933. Source: topic pocock-shipping-breakdown (PR backfilled when opened).

What to build

Adjudicate and (where accepted) land five upstream candidates in work-items:decompose and its references (evidence: topic memory slice upstream-bringover-audit.md):

• C5 prefactor look-ahead at decomposition time ("make the change easy, then make the easy change"); prefactor tickets become blockers
• C6 context-window slice sizing as a granularity calibration bar (each slice sized to one fresh context window) alongside S/M/L
• C7 integration-branch fallback for wide refactors: when expand-contract batches can't land green alone, share an integration branch all blocking a final integrate-and-verify ticket
• C8 "work the frontier" phrasing in the report step
• C17 PR-variant agent brief: verify whether reference/agent-brief.md covers PR-shaped briefs (current-behavior-of-the-diff, finish-what-exists); add if missing

Record each verdict in the upstream SSOT.

Acceptance criteria

• Each of C5/C6/C7/C8/C17 has an adopt/reject verdict recorded in the SSOT
• Accepted items landed in decompose (or agent-brief reference) with tests/lint green
• No behavior regressions to publish flow (approval gate, born-triaged, blockers-first ordering intact)

Blocked by

None — can start immediately.