Update agent instructions so that references to `documentation/wiki` are no longer necessary

[alexander-yevsyukov]

We have documentation with general coding and procedure guidelines for humans.

Current state

• Our agent instructions reference these guidelines in the hope that it would help agents.
  In reality, these guidelines are too far away from agents to be taken seriously.
• These guidelines are hardly verifiable in an automated way if they are just wiki pages. AI skills solve such things already better.
• We use vibe coding more and more. The guidelines need to be oriented to AI agents with humans being in control for verification and guidance.

Proposed improvements and actions

• Gradually migrate the content of our wiki for humans eliminating the content duplication.
• If there is something described under wiki is also described under agents, the duplication must be eliminated making wiki link to agents, instead of having the same or similar words around the topic.
• Let's start with obvious things like:
  • https://github.com/SpineEventEngine/documentation/wiki/Protobuf-code-style
  • https://github.com/SpineEventEngine/documentation/wiki/Kotlin-code-style
  • https://github.com/SpineEventEngine/documentation/wiki/Java-code-style
• Track the progress and suggested further steps under task documents which should link to this meta-issue.
• Update this issue description, if needed. Track progress in comments.

Verification

• When this issue is fully resolved, there should be no links to wiki from the content of this repository in the master branch.
• During the migration process, a page at wiki should link to corresponding document under agents when the page's content is processed.
• When the migration is complete the wiki should have one page with sections, brief descriptions and links to agents.

[alexander-yevsyukov]

The agents-side migration is complete in draft PR #15: all wiki pages with still-current contributor guidance now live under guidelines/, and no file in this repository (outside the migration ledger) references the wiki anymore.

The wiki-side work is intentionally not done yet. The task document .agents/tasks/wiki-content-migration.md carries the page-by-page disposition table, the checklist of deferred wiki edits (per-page back-links and the final single-index Home page), and archive proposals for the outdated pages (IntelliJ-IDEA-configuration, Build-artifacts, Validating-Builders) that need an owner decision. That checklist is to be executed after the PR lands on master, so every link target exists; then this issue can be verified and closed.

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