fix(corpflow): State the build/test and worktree contracts in CORPFLOW.md

[ikorich]

Motivation

CORPFLOW.md is the only file in this plugin that knows corpflow exists — the single thing a sibling agent reads on entry to learn the stage contract. A four-platform, eleven-stage stress test surfaced four gaps in it. Each produced observable cross-agent divergence; one was an outright contradiction of corpflow's own rules.

Changes

Gap	Before	After
Build/test	absent; android's copy pointed at raw Gradle, which agents/developer.md forbids	## Build and test (BINDING) — use the plugin's build-test command, --no-test for build-only, and the two correct outcomes when the gate still denies
Worktree	absent; "nothing outside .context/ is yours to create" — wrong for DV	## Worktree isolation (DV) — mismatch means stop and report; never create/move worktrees; re-confirm the pin before each write batch
Evidence manifest	unnamed	screenshots.md is the record of authority; do not read/write facts.screenshots
Return summary	role implicit	returning is what settles the stage; out-of-band reporting does not

Additive only — no restructuring. Section order is now Evidence → Build and test → Worktree → Artifacts.

What went wrong without these

• Two streams denied by the test-execution gate both invented --build-only (not a real flag on any plugin's command, so it classifies as a full test run and is denied again), and both fell back to direct toolchain calls.
• Two agents in identical worktree situations resolved the tree check differently — one blocked correctly, one relocated its tree and staled every guard keyed to metadata.workspace_path.
• One stream discovered the capped, last-writer-wins evidence ledger unaided and invented its own key to avoid erasing a sibling's rows.
• One completed stage with its artifact on disk sat at in_progress because it reported out of band.

Test plan

Documentation-only; no executable behaviour. Verified:

• All six sibling CORPFLOW.md files patched identically; plugin name correctly substituted into each build-test reference (/apple-developer:build-test, /frontend-developer:build-test, …).
• android's contradicting sentence removed — grep -c 'use scoped .Bash(gradle' → 0 — and replaced with a pointer to the build-test command, retaining scoped Bash(adb:*) for evidence capture.
• Section order and heading structure confirmed intact; file grew 253 → 302 lines with no existing section altered except the ## Artifacts scope sentence and android's evidence line.

Notes

The ## Artifacts line was corrected rather than deleted: .context/ remains the artifact home, and DV additionally owns its assigned platform tree inside its worktree.