Remove committed IDE and agent artifacts from repository

[cursor]

Problem

Non-source artifacts are committed to the repo:

• .idea/ — JetBrains Rider IDE settings
• .agents/ — internal agent task tracking files (e.g. FEAT-001 multi-provider refactor)

These are machine/tool-specific and add noise to diffs and clones.

Proposed solution

1. Add .idea/ and .agents/ to .gitignore
2. Remove tracked files from git history going forward (git rm -r --cached)
3. Document recommended IDE setup in README or CONTRIBUTING.md if needed

Acceptance criteria

• .idea/ and .agents/ are gitignored
• Directories removed from tracked files
• No functional code depends on these paths

Notes

Do not delete local copies on contributor machines; only stop tracking in git.