docs: add unified CHANGELOG.md

[Kamilbenkirane]

Add single CHANGELOG.md at repo root following Keep a Changelog format. Covers v0.9.1 through v0.11.0. Replaces CHANGELOG_V1.md.

[claude]

Code review

Issue: Subsection headings use H2 instead of H3

All category subsections within version entries use ##, the same level as the version headings themselves (## [0.11.0], ## [0.9.1]). The Keep a Changelog format, which this file claims to follow, requires version sections at H2 and child categories at H3.

This creates ~30+ headings structurally indistinguishable from version entries. Changelog parsers and GitHub's rendered TOC will treat subsections like ## 1. Tool Calling, ## Packaging Rationale, ## Tests, etc. as top-level releases rather than subcategories.

Affected area:

celeste-python/CHANGELOG.md

Lines 15 to 17 in 3e139c7

	## 1. Tool Calling (v0.11.0 — headline)

(and all ~30 similar headings throughout both version sections)

Fix: Change all subsection headings within version entries from ## to ###.