refactor: adopt the .abcd/development namespace for developer docs (v0.7.1)

[REPPL]

v0.7.1 — adopt the .abcd/development namespace for developer docs

Aligns ferry with the sibling abcd-cli repo so that docs/ is user-facing only
(the four Diátaxis directories). Developer-facing records move out of docs/ into a
new .abcd/development/ namespace, and ferry's repo scaffolding now stamps that layout.

Moves

• docs/plans/, docs/decisions/ → .abcd/development/{plans,decisions} (tracked renames)
• research → .abcd/development/research/ (incl. the CLI-surface and docs-structure studies)
• docs/ now holds only tutorials/ how-to/ reference/ explanation/ (+ assets, README)

Scaffold generator (the cross-repo bit)

internal/agents/scaffold.go now creates .abcd/development/{plans,research,decisions}
instead of docs/* for those dev dirs — so every repo ferry scaffolds gets the new
layout. Golden tests updated. This is why it's a user-facing "Changed", not a silent refactor.

Tooling + docs repointed

check-plan-shipped.sh, release.yml, consistency-lint.sh, docs/README.md,
CONTRIBUTING.md, both release/scaffold how-tos, AGENTS.md. Added .abcd/ structure READMEs.

Verification

gofmt/vet/build clean; go test ./... (incl. scaffold goldens) + full evals green;
consistency-lint, check-plan-shipped v0.7.1 (plan marked shipped), and docs-currency-lint
all pass; git grep shows no live docs/{plans,research,decisions} references.

Scope / follow-ups

• Working memory (.work/, .work.local/) stays at the repo root — folding it under
  .abcd/ is a small phase-2.
• The cross-repo ABCDevelopment/CLAUDE.md convention is updated to match (org-wide).
• Config-repo scaffold template (~/.config/ferry/repo/agents/templates/docs-README.md,
  ferry-managed, outside this repo) still describes the old docs/ dev-dirs — needs its map
  repointed + ferry apply. Flagged for a separate config-repo change.

After merge: scripts/release.sh v0.7.1 cuts the tagged release. Then v0.7.2 (the
Emacs/iTerm2/tmux hardening) is authored directly in .abcd/development/plans/.

Assisted-by: Claude:claude-opus-4-8