chore: remove stale inception docs PLAN.md and IDEA.md

[chaliy]

What

Delete PLAN.md and IDEA.md — both are stale inception documents that nothing in the repo references and that now actively mislead readers.

Why

I read both files end-to-end and grepped the repo for references. Findings:

IDEA.md (104 lines)

• The architecture diagram shows Anthropic and Gemini as first-class provider endpoints; the server actually implements only OpenAI and OpenResponses. New readers would assume features that don't exist.
• "Future Extensions" is half-done (WebSocket, stats endpoint — shipped) and half-aspirational (record/replay, Prometheus), same list as PLAN.md's Phase 6.
• Problem statement + design principles are the same content as the README.

PLAN.md (342 lines)

• Phase 7 "Set up GitHub releases" and "Publish to crates.io" are unchecked, but the project has shipped both (v0.2.x on crates.io with multi-platform release artifacts).
• The Technical Decisions Log decision Create README, license, and contribution files #3 declares "YAML for config" — already obsoleted by PR chore(config)!: migrate config file format from YAML to TOML #37's TOML migration.
• Unchecked phases (tool calling, Anthropic, Gemini) are roadmap items better tracked as GitHub issues than as a stale plan that the next agent has to re-evaluate from scratch.
• The "completed" phases duplicate information that's already in specs/architecture.md and the source tree.

Cross-repo grep

$ grep -rn "PLAN.md\|IDEA.md" --include="*.md" --include="*.rs" \
    --include="*.toml" --include="*.sh" --include="*.yml" --include="*.yaml" .
(no output)

Nothing links to either file.

Active substitutes are already in place

Need	File
Problem statement / "why"	README.md
Project structure	specs/architecture.md
API endpoints	specs/api-endpoints.md, specs/responses-api.md
Roadmap / future work	GitHub issues
Design decisions	Code comments (per AGENTS.md: "Keep decisions as comments on top of the file")

How

git rm PLAN.md IDEA.md. No code changes.

Risk

• Low. Nothing references either file. Anything still valuable in the Technical Decisions Log (e.g., "Axum over Actix-web", "tiktoken-rs", "atomic counters for stats") is already implicit in the code; if a specific decision needs preserving long-term, it belongs in the file it concerns per the AGENTS.md guideline.
• If anyone discovers a reason to resurrect a specific paragraph, git show HEAD~1:PLAN.md recovers it.

Checklist

• Verified no in-repo references with grep
• Confirmed each file's "still valid" content is duplicated elsewhere
• Listed the misleading entries that would harm a new reader