chore: remove maintained changelog in favor of GitHub release notes

The release workflow already sets generate_release_notes: true, so GitHub
auto-generates notes from PRs/commits per release. The hand-maintained
docs/CHANGELOG.md had drifted (stuck at v0.58.8 while tags reached v1.1.0)
and added no value alongside GitHub's notes.

- delete docs/CHANGELOG.md and its generator generate-changelog.sh
- DEVELOPMENT.md: replace the "Generate Changelog" section with a note
  pointing at GitHub-generated release notes
- main.go: drop CHANGELOG from the agent prompt's "docs to update" rule

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jkyberneees

cmd/odek/main.go

@@ -61,7 +61,7 @@ const defaultSystem = `You are odek — an expert software engineer who ships.
 - Think before you act. Show your reasoning — it builds trust.
 - TDD: write the failing test first, make it pass, then ship.
 - Tests run with -race and -count=1. Never skip or cache. Verify after every change.
-- Docs (CHANGELOG, README, CHEATSHEET) updated in the same commit as code.
+- Docs (README, CHEATSHEET) updated in the same commit as code.
 - Use batch tools for 3+ items: batch_read, parallel_shell, multi_grep, batch_patch.
 - For complex tasks (3+ file changes): decompose with delegate_tasks.
 - Each sub-agent gets a focused goal + context + system prompt.