fix(skills): remove broken default skills hub

[luoyanglang]

Summary

Issue: #297 — official skills hub URL returns 404
Type: Bug Fix
Severity: Medium

This PR removes the unavailable openclaw-ai/skills-hub repository from the default skill import path. The default import-official-hub command now only imports a verified reachable default skill (mmx_cli), while users who maintain their own Skills Hub can still opt in with OPENCLAW_SKILLS_HUB_BASE or ~/.openclaw/skills-hub-url.

Closes #297.

---

Root Cause

The default import path in scripts/skill_manager.py generated six URLs from https://raw.githubusercontent.com/openclaw-ai/skills-hub/main. That repository currently returns 404, so both add-remote examples and import-official-hub attempted to download unavailable files.

Source -> Sink:

• scripts/skill_manager.py default hub base -> OFFICIAL_SKILLS_HUB entries -> import_official_hub() -> add_remote() -> _download_file()
• README/docs examples -> users copy a raw URL from the unavailable hub -> _download_file()

# Before
OFFICIAL_SKILLS_HUB_BASE = 'https://raw.githubusercontent.com/openclaw-ai/skills-hub/main'
OFFICIAL_SKILLS_HUB = {
    'code_review': _get_hub_url('code_review'),
    ...
}

# After
OFFICIAL_SKILLS_HUB = {
    'mmx_cli': 'https://raw.githubusercontent.com/MiniMax-AI/cli/main/skill/SKILL.md',
}

---

Fix Description

Changed files:

• scripts/skill_manager.py:221 — removed the broken built-in hub base and mirror fallbacks.
• scripts/skill_manager.py:225 — added custom hub discovery via OPENCLAW_SKILLS_HUB_BASE or ~/.openclaw/skills-hub-url.
• scripts/skill_manager.py:240 — kept only verified mmx_cli as a default built-in skill.
• scripts/skill_manager.py:271 — kept import-official-hub compatible, but defaulted each skill to its own recommended agents when --agents is omitted.
• README.md:553 — replaced broken code_review examples with the reachable mmx_cli URL.
• docs/remote-skills-guide.md:160 — documented default skills and custom hub configuration.
• docs/remote-skills-quickstart.md — updated quickstart examples away from the 404 source.
• tests/test_skill_manager.py — added regression coverage for default URLs, custom hub opt-in, and recommended-agent import behavior.

Rationale:

• This avoids replacing one unavailable official source with an unverified third-party source.
• The existing CLI command name stays stable for compatibility.
• Users who have a private or community Skills Hub can still use the previous <base>/<skill>/SKILL.md layout explicitly.

---

Test Results

Test	Description	Status
python3 tests/test_skill_manager.py	Unit tests for default skill URLs, custom hub URLs, and recommended-agent import behavior	PASS
python3 -m py_compile scripts/skill_manager.py tests/test_skill_manager.py	Syntax check for changed Python files	PASS
OPENCLAW_HOME=/tmp/edict-issue-297-openclaw python3 scripts/skill_manager.py import-official-hub --agents menxia	Real CLI smoke test downloading the default mmx_cli skill	PASS
git diff --check	Whitespace check	PASS
rg ... openclaw-ai/skills-hub ...	Verified no default docs/code path still references the broken hub URL	PASS

Manual verification before the fix:

• curl -I https://github.com/openclaw-ai/skills-hub returned HTTP/2 404.
• curl -I https://raw.githubusercontent.com/openclaw-ai/skills-hub/main/code_review/SKILL.md returned HTTP/2 404.
• curl -I https://raw.githubusercontent.com/MiniMax-AI/cli/main/skill/SKILL.md returned HTTP/2 200.

---

Disprove Analysis

是否已有其他地方修复

Checked current open PRs before implementing. PR #298 and PR #299 do not touch remote skills. The earlier merged PR #287 added mmx_cli, but did not remove the broken openclaw-ai/skills-hub defaults or documentation references.

影响范围

Affected path is limited to remote skill import defaults and docs. Direct add-remote --source <url> continues to work for any valid HTTPS URL. Existing local remote skills already installed under ~/.openclaw are not migrated or deleted.

边界情况

When --agents is omitted, the command now imports each default/custom skill to its own recommended agents instead of building one combined agent list for every skill. If a user sets OPENCLAW_SKILLS_HUB_BASE or ~/.openclaw/skills-hub-url, the legacy skill names are restored using <base>/<skill_name>/SKILL.md.

已知局限

This PR does not introduce a replacement official Skills Hub. It intentionally keeps only verified reachable defaults and leaves custom hub selection explicit.

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Checklist

• 代码遵循项目现有风格（错误信息用中文，符合现有 CLI 惯例）
• 相关 issue 编号在 PR 描述中已引用（官方skills地址404 #297）
• 有测试覆盖（并包含真实 CLI 下载烟测）
• 无硬编码路径、密钥、调试输出