Remove /changelog command and all changelog-related infrastructure

[aebrer]

Summary

The /changelog slash command displays the contents of CHANGELOG.md, which is stale pi-mono content and hasn't been maintained for dreb. This is misleading to users. Since all dreb changes are tracked on GitHub anyway, the entire changelog subsystem should be removed to eliminate dead code and confusing UX.

Current Behavior

• /changelog renders outdated pi-mono release notes from CHANGELOG.md
• Startup shows a condensed or full changelog after version bumps
• Settings UI includes a collapseChangelog toggle
• settings.json tracks lastChangelogVersion to know what's "new"
• CHANGELOG.md files exist in multiple packages but contain no dreb-specific content

Proposed Behavior

• /changelog command is removed from slash command list
• No changelog display on startup
• collapseChangelog and lastChangelogVersion settings removed
• src/utils/changelog.ts and related imports deleted
• Documentation updated to remove all changelog references

Acceptance Criteria

• /changelog no longer appears in slash command autocomplete or responds to input
• Startup header never shows changelog content (condensed or full)
• collapseChangelog and lastChangelogVersion removed from Settings interface, SettingsManager, and settings.json serialization
• collapseChangelog toggle removed from /settings UI component
• src/utils/changelog.ts deleted; all imports removed
• README.md (root and package) no longer mentions /changelog
• docs/settings.md no longer documents collapseChangelog
• CHANGELOG.md files removed from all packages (or left as-is if still referenced by packaging — check package.json files array)
• Tests pass; no stale references remain

Context

The CHANGELOG.md files contain pre-fork pi-mono history. All dreb changes are available via GitHub releases and commit history.

Technical Notes

Key files to modify or delete:

• packages/coding-agent/src/modes/interactive/interactive-mode.ts — remove handleChangelogCommand(), getChangelogForDisplay(), changelogMarkdown field, startup display logic, /changelog slash command handler
• packages/coding-agent/src/modes/interactive/components/settings-selector.ts — remove collapseChangelog from props and onCollapseChangelogChange callback
• packages/coding-agent/src/core/settings-manager.ts — remove lastChangelogVersion and collapseChangelog from Settings interface and manager methods
• packages/coding-agent/src/utils/changelog.ts — delete entire file
• packages/coding-agent/src/config.ts — remove getChangelogPath() if no longer used elsewhere
• packages/coding-agent/README.md and root README.md — remove /changelog from slash command list
• packages/coding-agent/docs/settings.md — remove collapseChangelog row