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/issue-driven-orchestration.md

@@ -173,11 +173,10 @@ scope for this guide.
 
 ## Related
 
-- [docs/USER-GUIDE.md](USER-GUIDE.md) — task-oriented walkthroughs of
-  individual commands referenced above.
-- [docs/COMMANDS.md](COMMANDS.md) — full reference for `/gsd-*`
-  commands.
-- [docs/FEATURES.md](FEATURES.md) — feature-level capability matrix
-  (workspaces, manager, autonomous, verify, review, ship).
-- [docs/ARCHITECTURE.md](ARCHITECTURE.md) — phase-artifact lifecycle
-  and `STATE.md` mechanics.
+- [The phase loop](explanation/the-phase-loop.md) — how discuss → plan → execute → verify → ship fits together as a repeating cycle.
+- [Workspaces how-to](how-to/work-in-parallel-with-workstreams.md) — step-by-step guide to creating and managing parallel worktrees.
+- [docs index](README.md) — full table of contents for GSD Core documentation.
+- [docs/USER-GUIDE.md](./USER-GUIDE.md) — task-oriented walkthroughs of individual commands referenced above.
+- [docs/COMMANDS.md](COMMANDS.md) — full reference for `/gsd-*` commands.
+- [docs/FEATURES.md](FEATURES.md) — feature-level capability matrix (workspaces, manager, autonomous, verify, review, ship).
+- [docs/ARCHITECTURE.md](ARCHITECTURE.md) — phase-artifact lifecycle and `STATE.md` mechanics.