Add AGENTS.md maintenance protocol and Dashboards/QAN component guides

[ondrejpatocka]

Summary

• Add a "Maintaining This Document" section to AGENTS.md that instructs AI agents
  to review and update the file whenever they make structural changes (new components,
  tech stack changes, convention changes, etc.)
• Add component guides for Grafana dashboards (dashboards.instructions.md) and the
  QAN Grafana plugin (qan-app.instructions.md)
• Register both new guides in the AGENTS.md component tables
• Remove the stale percona/grafana-dashboards external repo reference (dashboards
  now live in-repo under dashboards/)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.79%. Comparing base (38c9b56) to head (c20571b).
⚠️ Report is 4 commits behind head on v3.

Additional details and impacted files

@@           Coverage Diff           @@
##               v3    #5226   +/-   ##
=======================================
  Coverage   47.79%   47.79%           
=======================================
  Files         410      410           
  Lines       41981    41981           
=======================================
  Hits        20066    20066           
+ Misses      19940    19939    -1     
- Partials     1975     1976    +1     

Flag	Coverage Δ
admin	35.80% <ø> (ø)
agent	53.36% <ø> (+0.01%)	⬆️
managed	47.71% <ø> (-0.01%)	⬇️
vmproxy	72.09% <ø> (ø)

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[ademidoff]

That whole comment is consuming tokens and is mostly irrelevant. One can easily figure out the revision date by the file timestamp.

As a general rule, we'd better keep such files tiny than fat. Letting AI figure it out is the best advice one can get nowadays :)

[ondrejpatocka]

Good catch, removed!
Makes sense to make Agents.md file as much accurate and condensed as we can (better to remove anything not relevant / not important / misleading).