Consolidate UNRELEASED changelog entries and add changelog guidance

[pjbull]

Trims the ## UNRELEASED section of HISTORY.md to user-facing changes and codifies changelog conventions for agents.

• HISTORY.md: replaces five detailed entries, most about test suite/CI internals, with two lines:
  • the one user-facing fix (Fix equal-mtime uploads and flaky timestamp tests #586): spurious OverwriteNewerCloudError when a write leaves the cache file's mtime equal to the cloud file's, with the scenario where users see different behavior;
  • one consolidated line for test suite reliability and faster live-backend CI (Fix equal-mtime uploads and flaky timestamp tests #586, Split cache cleanup test assertions with weakrefs #587, Tighten LocalClient download retries for dest-directory races #588, Speed up live backend tests in CI #589, Speed up live backend tests in CI #590).
• AGENTS.md: adds section 9 with the Keep a Changelog 1.1.0 guiding principles and change types (Added/Changed/Deprecated/Removed/Fixed/Security). It asks for entries that start with those verbs, grouping by type, one line for internal-only changes, and a concrete before/after scenario for fixes. Section 8's PR checklist now points to it.

Docs-only. No issue, and no HISTORY.md entry of its own because it is an internal change.

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[github-actions]

🚀 Deployed on https://deploy-preview-591--gallant-agnesi-5f7bb3.netlify.app

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.7%. Comparing base (ee8a620) to head (c571aa1).

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #591   +/-   ##
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  Coverage    94.7%   94.7%           
======================================
  Files          28      28           
  Lines        2297    2297           
======================================
  Hits         2176    2176           
  Misses        121     121           

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