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/README.md

@@ -7,7 +7,7 @@ This is the entry point to all project documentation. It is maintained automatic
 - Documentation is written in **English**.
 - One topic per file. Group related files into a folder when there are three or more.
 - Cross-reference with relative links so they keep working in GitHub, IDEs, and exported sites.
-- For architectural decisions, do **not** add them here — they belong in [../decisions/](../decisions/) as MADR 4.0.0 ADRs.
+- For architectural decisions, do **not** add them here — they belong in [decisions/](decisions/) as MADR 4.0.0 ADRs.
 
 ## Sections