feat(ask): add capability context — command surface, open PRs, live milestones

[snipcodeit]

Summary

• Adds new load_capability_context step to mgw:ask that builds a compact command surface index from commands/*.md front matter, fetches open PRs via gh pr list, and fetches live GitHub milestones via gh api
• Injects all three into a <mgw_capabilities> block in the classification agent prompt, giving it full awareness of MGW's command surface and live project state
• No new files, no lib changes — purely additive context in a single command file

Closes #144

Milestone Context

• Milestone: v3.5 — Foundation Hardening
• Phase: quick task — single file, no phase plan required

Changes

commands/ask.md

• New step load_capability_context inserted between load_project_context and load_active_state
• <mgw_capabilities> block (${COMMAND_SURFACE}, ${PR_CONTEXT}, ${MILESTONE_LIST}) added to Task() prompt after <recent_changes>, before <classification_rules>
• <context> header updated to document the two new sources
• success_criteria extended with 3 new checklist items

Test Plan

• Run mgw:ask with a capability question (e.g. "does mgw:run handle --dry-run?") — verify agent response references the command surface
• Run mgw:ask with no .mgw/project.json present — verify milestone list falls back to live GitHub API data instead of empty
• Run mgw:ask with an open PR in flight — verify agent context includes the PR
• Run mgw:ask with a standard issue-classification question — verify no regression in classification behavior (issue/active-state context unchanged)