CHANGELOG: split Unreleased into published alpha.8 and post-release

[LinuxDevil]

Splits CHANGELOG [Unreleased] into what is in the published @lousho/build-ai-agent@1.0.0-alpha.8 (npm, 2026-10-02, from 1574d28) and what is on main only. Entry text unchanged; done with a script comparing against git show 1574d28:CHANGELOG.md.

• [Unreleased]: 57 entries (Security 1, Changed 7, Added 24, Fixed 6, Tests 2, Docs 17), plus one sentence saying it lists what is on main and not on npm.
• [1.0.0-alpha.8] - 2026-10-02: 144 entries; the older alpha.8 section (2025-10-05, never published) is folded into Added. The telegramChannel entry moved to Unreleased.
• Total 201 before and after, per heading.
• One entry edited after release (only the docs URL, mintlify to lousho.com): kept under alpha.8 Published.
• llms files regenerated: no content change.

Verification: npx vitest run docs scripts (76 passed), npm run docs:llms:check ok.

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