docs: clarify Codex usage surfaces

[YSCJRH]

Summary

• Clarifies the three Codex usage surfaces: record-only Workday plugin, read-only MCP, and development threads.
• Adds English and Chinese README guidance plus focused Codex App / plugin / MCP docs tables.
• Bumps package/plugin metadata to v0.2.28 for the Phase 4 patch release candidate.

Validation

• python -m pip install -e ".[dev]" -> installed winchronicle==0.2.28
• python -m pytest -q -> 316 passed, 1 skipped
• python harness/scripts/run_mcp_smoke.py -> PASS: MCP stdio smoke passed
• python -m winchronicle codex install --dry-run -> six fixed read-only tools only
• git diff --check -> clean
• $env:GITHUB_HEAD_REF="codex/p4-codex-usage-card"; python harness/scripts/check_productization_scope.py -> Phase p4 scope check passed for 10 changed file(s).

Isolated review reports

• Privacy Boundary Reviewer: PASS. No capture-surface or MCP write-tool expansion; Workday/development/MCP separation is explicit.
• Product Clarity Reviewer: PASS. English and Chinese docs distinguish record-only plugin, read-only MCP, and development thread.
• Release Gate Reviewer: PASS. Changed files are Phase 4 docs plus release metadata; version metadata is consistent at 0.2.28; required gates have fresh passing evidence.

Privacy boundary

No screenshots, OCR, keyboard logging, clipboard capture, audio, cloud upload, desktop control, background daemon/service, infinite polling, MCP write tools, arbitrary file reads, network upload, or product-targeted capture behavior were added. MCP remains read-only with the existing six tools.