chore: relocate ADRs from decisions/ to docs/decisions/ to consolidate documentation

Move the ADR folder under docs/ alongside the rest of the project
documentation. Convention (flat folder, globally-sequential 4-digit
numbering, tags-based categorization, MADR 4.0.0 format) is unchanged
- only the path moved.

- git mv decisions docs/decisions preserves history for all 18 ADRs +
  README + template (19 files renamed in this commit).
- ADR-0001 amended in-place with a dated note documenting the
  relocation. Status remains 'accepted' - the location detail
  changed, the decision did not.
- All cross-references updated:
  - CLAUDE.md (~17 ADR links + 3 mentions of decisions/ in the Project
    rules section)
  - docs/README.md (now references decisions/ as a sibling under docs/)
  - docs/setup/03-angular-nx-monorepo.md (paths shortened from
    ../../decisions/ to ../decisions/, since setup/ and decisions/ are
    now both inside docs/)
  - docs/decisions/0003 ../CLAUDE.md adjusted to ../../CLAUDE.md
    (one extra level of nesting)
  - docs/decisions/template.md mention of the README path
  - notes/asvs-level-decision-briefing-rssi.md mention of the index

Sanity verified: every ADR link in CLAUDE.md, docs/setup/03, and
docs/decisions/0001 resolves to an existing file. pnpm nx run-many
-t lint passes on 8 projects.

docs/decisions/template.md

@@ -0,0 +1,80 @@
+---
+# `tags` is REQUIRED for this project — pick from the vocabulary in docs/decisions/README.md.
+# The other fields are standard MADR 4.0.0 metadata; remove any that don't apply.
+status: '{proposed | rejected | accepted | deprecated | … | superseded by [ADR-NNNN](NNNN-example.md)}'
+date: { YYYY-MM-DD when the decision was last updated }
+decision-makers: { list everyone involved in the decision }
+consulted:
+  {
+    list everyone whose opinions are sought (typically subject-matter experts); two-way communication,
+  }
+informed: { list everyone who is kept up-to-date on progress; one-way communication }
+tags: [<tag1>, <tag2>]
+---
+
+# {short title, representative of solved problem and found solution}
+
+## Context and Problem Statement
+
+{Describe the context and problem statement, e.g., in free form using two to three sentences or in the form of an illustrative story. You may want to articulate the problem in form of a question and add links to collaboration boards or issue management systems.}
+
+<!-- This is an optional element. Feel free to remove. -->
+
+## Decision Drivers
+
+- {decision driver 1, e.g., a force, facing concern, …}
+- {decision driver 2, e.g., a force, facing concern, …}
+- …
+
+## Considered Options
+
+- {title of option 1}
+- {title of option 2}
+- {title of option 3}
+- …
+
+## Decision Outcome
+
+Chosen option: "{title of option 1}", because {justification, e.g., only option which meets k.o. criterion / resolves force / comes out best (see below)}.
+
+<!-- This is an optional element. Feel free to remove. -->
+
+### Consequences
+
+- Good, because {positive consequence, e.g., improvement of one or more desired qualities, …}
+- Bad, because {negative consequence, e.g., compromising one or more desired qualities, …}
+- …
+
+<!-- This is an optional element. Feel free to remove. -->
+
+### Confirmation
+
+{Describe how the implementation of / compliance with the ADR is confirmed. E.g., by a review, an ArchUnit test, a Lighthouse CI threshold, an axe-core assertion. Although classified as optional, this element is included in most ADRs.}
+
+<!-- This is an optional element. Feel free to remove. -->
+
+## Pros and Cons of the Options
+
+### {title of option 1}
+
+{example | description | pointer to more information | …}
+
+- Good, because {argument a}
+- Good, because {argument b}
+- Neutral, because {argument c}
+- Bad, because {argument d}
+
+### {title of option 2}
+
+{example | description | pointer to more information | …}
+
+- Good, because {argument a}
+- Good, because {argument b}
+- Neutral, because {argument c}
+- Bad, because {argument d}
+
+<!-- This is an optional element. Feel free to remove. -->
+
+## More Information
+
+{Additional evidence/confidence for the decision; team agreement; when and how the decision should be realized; consequences for other decisions, projects, or teams; links to related ADRs.}