docs: rebrand to GSD Core and restructure docs with Diataxis (#605)

* chore: wire docs/agents config into AGENTS.md Agent skills section

Add the `## Agent skills` discovery block pointing the engineering
skills at the existing docs/agents/{issue-tracker,triage-labels,domain}.md
files (issue tracker, triage label mapping, single-context domain docs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs: rebrand to GSD Core and restructure docs with Diataxis

Reorganise the root README and docs/ around the Diataxis framework
(tutorials, how-to guides, reference, explanation), add new how-to
guides and schema references (STATE.md / CONTEXT.md / PLAN.md /
planning artifacts), and cross-link the whole set. Update the lone
legacy gsd-build reference to open-gsd; keep internal get-shit-done/
filesystem paths unchanged (directory rename tracked separately in
open-gsd/gsd-core#604). Regenerate the ja-JP, ko-KR, pt-BR and zh-CN
localised trees to mirror the new structure.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* docs: backfill changeset PR number (#605)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/adr/22-plan-drift-guard.md

@@ -74,7 +74,7 @@ Locked sub-decisions:
 - **Hard-block on any MISSING (as originally proposed).** Rejected for rung 0–1: false positives from dynamic/re-exported/generated symbols would block valid plans and get the default-on guard switched off. Retained only for rung >=3.
 
 ## References
-- Issue: open-gsd/gsd-core#22 (migrated from gsd-build/get-shit-done#3813)
+- Issue: open-gsd/gsd-core#22 (migrated from open-gsd/gsd-core#3813)
 - Relates to #3802 (GitNexus first-class code intelligence) — rung 4 backend
 - arXiv:2409.20550 — hallucination taxonomy + RAG mitigation (modest gains)
 - arXiv:2502.05111 — grammar-constrained decoding (soft vs hard constraints)