Remove agent harness, simplify to portable Claude Code only

[Copilot]

The repo was cluttered with harness-first planning infrastructure (ExecPlans, checklists, multi-provider Codex support) that added friction without value for the core use case: copy-to-any-Windows-PC Claude Code.

Deleted

• AGENTS.md, HARNESS_CHECKLIST.md, CODEX_EXECUTION_GUIDE.md — harness control files
• All of docs/ — 15 planning/ops docs + entire exec-plans/ tree

Simplified to Claude-only

• scripts/adapters/catalog.json — removed codex entry
• profiles/profiles.json — removed openai profile, anthropic only

Rewrote launcher (scripts/pcoder.cjs, ~1,486 → ~700 lines)

• Dropped all Codex-specific code paths and settings keys
• pcoder with no args now launches Claude in the current directory directly
• Improved error messages with actionable install hints and mode-switch suggestions

Rewrote README.md

Replaced harness-oriented docs with a minimal quick-start covering the two real workflows:

REM OAuth (portable, credentials stored in state/auth/)
scripts\pcoder setup --init
scripts\pcoder auth login
scripts\pcoder

REM API key
set ANTHROPIC_API_KEY=sk-ant-...
scripts\pcoder setup --claude-auth api
scripts\pcoder

Portability behavior (OAuth creds in state/auth/, no registry writes, no system PATH changes) is unchanged.

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