docs: add three-path onboarding

[YSCJRH]

Summary

• Add a top-level Demo / Workday / MCP path chooser to the English and Chinese READMEs.
• Move the full installation sequence behind a developer-install heading so first-time users can choose the smallest path first.
• Clarify Windows first-run, Codex Workday, and MCP setup boundaries, including dry-run behavior and metadata-only MCP usage.
• Bump patch metadata to 0.2.26 for the Phase 2 productization release.

Validation

• python -m pip install -e ".[dev]"
• python -m pytest -q -> 316 passed, 1 skipped
• python harness/scripts/run_mcp_smoke.py -> PASS
• python -m winchronicle codex install --dry-run -> PASS
• GITHUB_HEAD_REF=codex/p2-three-path-onboarding python harness/scripts/check_productization_scope.py -> PASS for 10 changed files
• git diff --check origin/main...HEAD -> PASS

Isolated Review Reports

• Privacy Boundary Reviewer: PASS. The onboarding docs keep dry-run, metadata-only, finite Workday, observed-content, and read-only MCP boundaries clear.
• Product Clarity Reviewer: PASS. README now presents Demo / Workday / MCP paths before the full developer install; docs link each user type to the next step.
• Release Gate Reviewer: PASS. Scope is limited to P2 docs plus release metadata; version metadata is consistent at 0.2.26.

Privacy Boundary

This PR changes onboarding docs and release metadata only. It does not add or imply default screenshots, OCR, clipboard capture, keylogging, audio recording, cloud upload, desktop control, background daemon/service, infinite polling, MCP write tools, arbitrary file reads, network upload, product-targeted capture behavior, or official OpenAI affiliation. Observed UI content remains untrusted observed data.