chore: release v0.3.0 — checkpoint/lock reliability, provider gate, docs update

[beersoccer]

Summary

• Bump version to v0.3.0 across manifest.yaml, pyproject.toml
• CHANGELOG.md: add v0.3.0 release entry
• README.md: update title, last-updated date, What's New section, version history row
• CONFIG.md: update last-updated date and .difypkg filename reference
• PR_TEMPLATE.md: refresh for v0.3.0 Dify Marketplace submission
• .cursor/AGENTS.md: sync tool count to 12, add complete utils/ module list, add full test directory structure + design notes (471 unit tests)
• .cursor/SPEC.md: add v0.3.0 acceptance criteria section

What's in v0.3.0 (already merged via PR #44)

• Checkpoint reliability: async-mode resume cursors now fully restored; save order switched to add-first-then-delete to prevent accidental checkpoint loss
• Distributed lock reliability: read-after-write verification after acquire; forget_memories cleans up expired lock records
• Provider compatibility gate: 117 parametrized unit tests covering canonical provider names and config fields for all mainstream mem0 providers; 28 cross-check the live factory registry to catch version-upgrade drift; removed invalid mistral provider entry
• Test isolation: async checkpoint tests refactored to thread-isolated sync wrappers, eliminating pytest-asyncio AUTO-mode cross-test contamination
• Test suite: 471 unit tests passing (up from ~380)

Test plan

• pytest tests/unit/ -q → 471 passed
• Version strings consistent across manifest.yaml, pyproject.toml, CHANGELOG.md, README.md, CONFIG.md
• No stale 0.2.12 references in versioned files
• AGENTS.md tool count = 12, test count = 471

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