feat(channels): add TikTok via OpenCLI

[misako-335]

Summary

• add a TikTok channel backed by the existing OpenCLI browser-session integration
• register TikTok in doctor and add a tiktok installer alias that resolves to OpenCLI
• route TikTok URLs, including www, mobile, and vt short links, with exact host-boundary checks
• document read-only profile, search, user, explore, and signed-in creator-metrics commands in the bundled skills and install guide
• update the platform count and Chinese/English capability docs

Safety boundaries

• Agent Reach remains an installer, doctor, and router; agents call OpenCLI directly
• doctor does not execute a TikTok platform command or claim a verified login session
• the bundled guidance lists only read-only commands and explicitly excludes comment, follow, and like operations
• creator-videos is documented as data for the currently signed-in TikTok Studio account, not arbitrary creators

Validation

• ruff check agent_reach tests
• mypy agent_reach
• pytest: 587 passed with the repository constraint set
• focused channel, CLI, and URL tests: 159 passed
• wheel build and smoke inspection: TikTok channel included, no duplicate entries

Upstream capability source

• OpenCLI v1.8.6 release: https://github.com/jackwener/OpenCLI/releases/tag/v1.8.6
• OpenCLI v1.8.6 TikTok adapter docs: https://github.com/jackwener/OpenCLI/blob/v1.8.6/docs/adapters/browser/tiktok.md