Improve documentation discoverability and cross-linking

[JAORMX]

Summary

Our documentation was well written but hard to navigate: the architecture
docs — the richest knowledge in the repo — were only reachable through
passing inline-text mentions, several READMEs were not linked from any hub
(so a reader browsing the docs could not discover them), and the numbered
architecture documents rarely linked to each other. This makes it harder for
both new contributors and AI coding assistants to move from an entry point to
the document they need.

This PR improves discoverability and cross-linking without changing any doc
content:

• Entry points → architecture. CLAUDE.md now opens with a prominent,
  clickable pointer to the architecture index and a suggested reading order.
  AGENTS.md is added as a symlink to CLAUDE.md so tools that follow the
  AGENTS.md convention get the same canonical guidance (one source of truth,
  no drift).
• Reconnect standalone READMEs. Link the generated CLI reference, the
  server API docs, the e2e test guide, and the local Keycloak and
  OpenTelemetry setups from the most relevant developer guide. Add a
  Contributing section to the Helm chart README templates so their
  CONTRIBUTING guides are reachable (regenerated via helm-docs).
• Cross-link the architecture docs. Add the missing lateral "Related
  Documentation" links between strongly related documents (deployment modes ↔
  groups, runconfig ↔ registry, transport ↔ operator, skills ↔ plugins), and
  give 14-plugins-system.md the Related section it was missing.

Type of change

• Documentation

Test plan

• Manual testing (describe below)

Verified that every relative link added or changed resolves to an existing
file. The two Helm chart README.md files are generated — I edited the
README.md.gotmpl templates and regenerated with helm-docs, so the
generated output stays in sync (diff is limited to the new Contributing
section).

Does this introduce a user-facing change?

No. Documentation navigation only; no doc content or product behavior changes.

Special notes for reviewers

• AGENTS.md is intentionally a symlink to CLAUDE.md rather than a separate
  file, to avoid two agent-instruction files drifting apart.
• The Helm chart README.md changes are generated output; the source of truth
  is the README.md.gotmpl change in the same commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.78%. Comparing base (9662681) to head (5de2080).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5739      +/-   ##
==========================================
+ Coverage   70.74%   70.78%   +0.04%     
==========================================
  Files         683      683              
  Lines       69194    69194              
==========================================
+ Hits        48950    48982      +32     
+ Misses      16663    16616      -47     
- Partials     3581     3596      +15     

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