docs(adrs): place ADRs by decision scope (#2116)

[josecelano]

Summary

• Add a root ADR that places decisions by architectural scope: package-owned decisions remain with extractable packages, while repository-wide, multi-package, and inter-package decisions belong in the root collection.
• Define package-local collection structure, separate index ownership, and the root-ADR supersession procedure.
• Synchronize ADR authoring, issue-authoring, template, and documentation navigation guidance.
• Record completed verification evidence in the issue specification.

Scope

• Touched: root ADR documentation, planning skills, ADR template, documentation navigation, and docs/issues/open/2116-adr-placement-policy.md.
• The existing UDP ADR is not moved; its migration remains owned by the separate UDP implementation work.

Validation

• linter markdown
• linter cspell
• linter all
• TORRUST_GIT_HOOKS_LOG_DIR=.tmp ./contrib/dev-tools/git/hooks/pre-commit.sh
• Manual review of root/package scope criteria, the tracker-client local index, root-index boundary, and local ADR supersession.

Closes #2116

[Copilot]

Pull request overview

This PR formalizes an ADR placement policy based on decision scope (root vs package-local) and propagates that policy across the project’s canonical documentation, templates, and planning skills so ADR ownership remains compatible with future package extraction.

Changes:

• Added a root ADR defining scope-based ADR placement, local collection requirements, and root supersession rules.
• Updated ADR guidance/navigation (docs index + ADR README/index) and the ADR template to reflect root vs package-local placement.
• Updated planning skills and the #2116 issue spec to require/record ADR placement expectations and verification evidence.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
docs/templates/ADR.md	Adds an explicit “Scope” section to the ADR template to drive correct collection placement.
docs/issues/open/2116-adr-placement-policy.md	Records completion status, verification evidence, and acceptance-criteria signoff for the ADR placement policy work.
docs/index.md	Updates documentation navigation wording to distinguish root ADR guidance/index.
docs/AGENTS.md	Updates artifact placement guidance to differentiate root vs package-local ADRs.
docs/adrs/README.md	Updates root ADR collection guidance to explain scope-based placement, index ownership, and supersession.
docs/adrs/index.md	Renames/clarifies the index as root-only and adds placement-rule pointer.
docs/adrs/20260830124000_place_adrs_by_decision_scope.md	Introduces the root ADR that defines the placement policy and supersession procedure.
.github/skills/dev/planning/create-issue/SKILL.md	Requires planned ADRs in issue specs to state expected root vs package-local placement by scope.
.github/skills/dev/planning/create-adr/SKILL.md	Updates ADR authoring workflow to include scope-based placement and collection-specific indexing.

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[josecelano]

ACK 8f622ec