feat(sdk): reduce context prompt sizes with truncation and cache-friendly ordering (#1615)

* chore: add v1.31.0 npm known-issue notice to issue template config

Adds a top-priority contact link to the issue template chooser so users
are redirected to the Discussions announcement before opening a duplicate
issue about v1.31.0 not being on npm.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(sdk): reduce context prompt sizes with truncation and cache-friendly ordering (#1614)

- Reorder prompt assembly in PromptFactory to place stable content (role,
  workflow, phase instructions) before variable content (.planning/ files),
  enabling Anthropic prompt caching at 0.1x input cost on cache hits
- Add markdown-aware truncation for oversized context files (headings +
  first paragraphs preserved, rest omitted with line counts)
- Add ROADMAP.md milestone extraction to inject only the current milestone
  instead of the full roadmap
- Export truncation utilities from SDK public API
- 60 new + updated tests covering truncation, milestone extraction,
  cache-friendly ordering, and ContextEngine integration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/ISSUE_TEMPLATE/config.yml

@@ -1,5 +1,8 @@
 blank_issues_enabled: false
 contact_links:
+  - name: "⚠️ v1.31.0 not on npm yet (known issue — workaround inside)"
+    url: https://github.com/gsd-build/get-shit-done/discussions
+    about: v1.31.0 was not published to npm due to a hardware failure. Read the pinned announcement for the workaround before opening an issue.
   - name: Discord Community
     url: https://discord.gg/gsd
     about: Ask questions and get help from the community