docs: show all integrations in READMEs and default installs to master

[happy-v587]

Which issue or RFC does this PR close?

Closes #.

Rationale for this change

The READMEs listed only a subset of the supported integrations and their installation commands: the quick start omitted pi, workbuddy, and (in the Japanese README) openclaw, the integration tables missed Bub, Pydantic AI, LangChain, and LangGraph, and no project logo was shown. The install instructions also split users across a pinned release (0.0.2) and master, while all setup commands and CLI defaults already target master.

What changes are included in this PR?

• Add the PowerContext logo to the header of all three READMEs (README.md, README_CN.md, README_JP.md) via a <picture> element that adapts to GitHub light/dark mode using the existing theme/powercontext/assets/images/ assets.
• Rewrite the quick-start install block to install the CLI/Server from git+...@master and list all eight powercontext setup commands (codex, claude-code, dsh, hermes, openclaw, opencode, pi, workbuddy) with --ref master, plus a powercontext setup select example. Remove the ==0.0.2 / --ref v0.0.2 release-path instructions and the OpenClaw/OpenCode special case.
• Expand each integration table to 12 tiles in two rows of six: add Bub (new docs/assets/bub.svg badge, WorkBuddy-style), Pydantic AI (github.com/pydantic.png avatar), LangChain and LangGraph (github.com/langchain-ai.png avatar). Also add the previously missing OpenCode tile to the Japanese README.
• Update the integration enumeration paragraphs and the Python-integration paragraph (LangChain middleware, LangGraph adapter, Pydantic AI middleware, Bub plugin) with matching links.
• Complete the powercontext doctor host list in the quick start.

Are there any user-facing changes?

• Documentation-only change; no API, CLI, or persisted-format changes. Users now see installation instructions that default every method to the master branch.

How was this change tested?

• Verified all referenced files exist (docs/en/docs/how-to/configure-{pydantic-ai,langchain,langgraph}.md, docs/zh/... equivalents, integrations/bub/README.md, docs/assets/bub.svg).
• Verified each integration table now renders 12 tiles and no 0.0.2 references remain in any README.
• Verified logo/avatar URLs resolve (GitHub org avatars return 200/302; local SVGs tracked in git).
• Ran uv run prek run --files README.md README_CN.md README_JP.md docs/assets/bub.svg — all hooks passed.

AI usage statement

Built with opencode (GLM, model glm-5.3-flash): edited the three READMEs and the new Bub badge, validated links and hook checks, and prepared this PR.

[happy-v587]

@AlexStocks PTAL

[Teingi]

LGTM