MAINT streamline repository guidance and contribution workflow

[BabaSanfour]

Summary

1. Added AGENTS.md as the repository-specific source of truth for architecture, scientific/test contracts, CI, public API ownership, and agent guidance.
2. Added guidance for agents helping users apply mne-denoise correctly, including current public APIs, container semantics, method assumptions, and experimental APIs.
3. Simplified CONTRIBUTING.md into a concise human contribution entry point.
4. Reworked README.md into a short user-facing package front door.
5. Added CLAUDE.md as a pointer to AGENTS.md.
6. Simplified the pull-request template and replaced Markdown issue templates with structured YAML forms.
7. No package behavior, public API, numerical behavior, scientific implementation, tests, dependency floors, or package architecture changed.

Validation

• spin lint
• spin docs

The full documentation build passed with warnings treated as errors. Scientific method documentation and callable docstrings were not audited in this PR; that work is reserved for PR 2.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.41%. Comparing base (ae59398) to head (7039107).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #115   +/-   ##
=======================================
  Coverage   85.41%   85.41%           
=======================================
  Files          77       77           
  Lines       10591    10591           
  Branches     1859     1859           
=======================================
  Hits         9046     9046           
  Misses        927      927           
  Partials      618      618           

Flag	Coverage Δ
unittests	85.41% <ø> (ø)

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