fix(devcontainer): converge mise to its pin, drop the deprecated Copilot extension, and name volumes per DEVC-08

[justinmerrell]

Follow-up to the dev container review done alongside #160. Tracks with musher-dev/engineering-conventions#65.

What changes

• mise converges to its pin. base_install_mise used to skip whenever any mise was on PATH. A container created before min_version moved to 2026.9.12 kept 2026.9.6, which refuses the config, so none of the pinned CLIs installed, and task check:conventions failed with mise version 2026.9.12 is required. It now replaces any mise that is not the pinned release. Both branches were tested: the matching version skips, and a stubbed 2026.9.6 is replaced.
• Deprecated extension removed. github.copilot is deprecated since VS Code 1.109, and github.copilot-chat provides the whole experience.
• Volumes renamed to the DEVC-08 form. musher-spec-${devcontainerId}-<purpose> becomes musher-${devcontainerId}-<purpose>, the form engineering-conventions EC-0027 DEVC-08 checks from 0.6.3 on. This repository pins 0.6.1 today, so nothing reports it yet, and the upgrade will not add a finding.

Before you rebuild: keep your volumes' data

A renamed volume starts empty. That means a new gh auth login, and a fresh ~/.claude and ~/.codex, including agent memory and history. To carry the data over, stop the container, then run this on the host in a POSIX shell (WSL on Windows) before rebuilding. ${devcontainerId} does not change, so each old volume maps to its new name:

for old in $(docker volume ls -q | grep '^musher-spec-'); do
  new="musher-${old#musher-spec-}"
  docker volume create "$new" >/dev/null
  docker run --rm -v "$old":/from -v "$new":/to alpine sh -c 'cp -a /from/. /to/'
  echo "copied $old -> $new"
done

Once the rebuilt container shows your data, remove the old volumes with docker volume rm <name>.

Checks

task check:shell, check:format and check:conventions pass. The Validate dev container workflow builds the container with --frozen-lockfile.

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