Share one composed workspace across CI jobs

[schickling-assistant]

Outcome

Compose the repository and resolve the devenv closure once per CI run. Publish that result as a job artifact or equivalent warm cache that downstream jobs restore.

Evidence

A measured canary spent 94.5 of 176.7 aggregate job-minutes (53.6%) in repeated composition and devenv resolution across 15 jobs. Five canaries consumed about 7.9 runner-hours in this repeated setup. The typecheck job spent 7.0 minutes composing and 0.3 minutes type-checking.

Acceptance

• one job owns composition and closure resolution
• downstream jobs restore committed-input-equivalent state instead of recomposing
• no job consumes untracked workspace state
• measured aggregate setup falls from about 90 job-minutes per run toward 10
• measured canary wall time falls from about 30 minutes toward 15
• cache credentials remain unavailable to pull-request jobs

Refs #1147
Refs #1227

Posted on behalf of @schickling

field	value
agent_identity	dev3.direct.omp.x7qqfv7y
session	dev3.x7qqfv7y
agent_persona	generalist
agent_supervisor	unavailable
agent_tool	OMP
agent_tool_version	18.1.7
agent_runtime	OMP 18.1.7
tooling_profile	dotfiles@c5db483