feat: enforce container environment boundaries

[webup]

Summary

• separate host, Docker-control, application-carrier, and Compose-substitution environments with bounded dotenv parsing and value-free failures
• materialize deterministic private build bundles, pass application values only through the final runtime carrier, and keep pip credentials in BuildKit secret mounts
• enforce the preloaded-v1 image/runtime manifest contract, isolated startup, exact runtime ownership, and fail-closed planning before container side effects
• add cross-platform private artifact cleanup, OCI/Docker image-layer scans, and hosted Docker acceptance on current Compose plus the supported 2.24.0 floor
• prepare the package version as 0.3.0

This supersedes #69: its shell-precedence goal is preserved within the broader typed container-boundary design.

Test Plan

• exact-head hosted CI at 1548c1d08d228a97a9136384afbef48fef967df3: https://github.com/ob-labs/agentseek-api/actions/runs/32400539402
  • 18/18 jobs successful after rerunning the sole failed job, a previously reproduced minute-59 cron ordering tie outside the Train B diff
  • current-runner Docker/Compose boundary, regression suite, generated up --wait, health/API smoke, and image/history scan
  • pinned Compose 2.24.0 equivalent proof
  • all three Redis durable-execution backends
  • Windows, macOS, and Ubuntu CLI compatibility
  • coverage-backed unit and integration suite
• final local non-Docker suite: 1465 passed, 10 skipped, 14 Docker-marked deselected, coverage 90.19%
• final affected boundary suite: 444 passed, 3 skipped
• Redis runtime script regressions: 6 passed
• uv lock --check --offline
• Ruff check/format, shell syntax, and git diff --check
• independent implementation-range review: APPROVE with no actionable findings; final hosted harness corrections are covered by the exact-head CI above

Target Branch Check

• feature/* PRs target develop
• release/* PRs target main
• hotfix/* PRs target main

[webup]

Thanks everyone—Train B is now squash-merged into develop at 9a415f1361f9acb856013f8922573d87e16d6eab. Exact-head hosted CI passed 18/18 jobs, including current Compose, Compose v2.24.0, Windows, and all Redis backends.