feat: add prototype mission board

[JOY (JOY)]

Summary

• Add a debug-labeled Gate mission board panel to the runtime HUD.
• Add the Ash Underpass Signal mission card with rank, objective, risk, TIME exposure, reward estimate, recommended roles, current body context, optional ally placeholder, and hidden objective placeholder.
• Add a local-only Mark Pending action that does not start a Gate run, grant rewards, spend TIME, or mutate quest state.
• Update ROADMAP and CHANGELOG.

Verification

• Local code-review fallback: APPROVED. Checked Unity UI scope, debug-only mission selection, no authoritative reward/TIME/quest mutation, no client model/provider calls, and no package changes.
• npx markdownlint-cli2@0.15.0 CHANGELOG.md ROADMAP.md: pass.
• git diff --check: pass.
• Unity MCP compile and Play Mode smoke will run on root dev after merge, because the active Unity Editor is attached to the root project rather than this worktree.

Closes #143