Community skill install fails for skills under non-standard repo paths (deep/renamed dirs)

[cravenceiling]

Some skills.sh skills fail to install because Houston can't locate their SKILL.md in the source repo. The marketplace then shows a "failed" pill without surfacing the real reason. Split off from #398 / PR #479 as a follow-up.

Repro

• anthropics/claude-code → writing-hookify-rules → install fails with SkillError::SkillNotInRepo ("Couldn't find that skill in the repo. The author may have renamed or removed it.").
• Near-miss that currently works only thanks to fuzzy matching: vercel/ai → ai-sdk actually lives at skills/use-ai-sdk/SKILL.md (directory use-ai-sdk ≠ skillId ai-sdk).

Root cause

houston_skills::remote::install_skill (engine/houston-skills/src/remote.rs) guesses the path from a small candidate list:

let candidates = [
    format!("skills/{skill_id}/SKILL.md"),
    format!("{skill_id}/SKILL.md"),
    "SKILL.md".to_string(),
];

then falls back to find_skill_path_in_repo, which fetches the recursive git tree and matches by (a) derived directory name == skillId, (b) path containing skillId as a substring + frontmatter name: == skillId.

This misses when:

1. The skill dir name doesn't match the skillId and isn't a substring (and frontmatter name differs).
2. The repo is large and the git tree is truncated: true — some SKILL.md paths never appear in the response (e.g. anthropics/claude-code is enormous), so the scan can't find them.
3. The skills.sh search result id is owner/repo/<skillId> where <skillId> is only the directory name, not the real sub-path, so we can't fetch directly.

Secondary: error not surfaced

ui/skills/src/community-skills-browser.tsx handleInstall catches the failure and sets a per-skill "failed" state but does not toast errorMessage(err) with a Report-bug affordance — so the user sees "failed" with no reason. Same shape in add-skill-dialog-store-view.tsx. This is the no-silent-failures policy (CLAUDE.md) leaking.

Suggested directions

• Best: get the real SKILL.md path from the skills.sh API (if it exposes it) and fetch it directly instead of guessing.
• Handle truncated trees: fetch subtrees lazily, or use the Git Contents/Search API to locate SKILL.md.
• Broaden candidate conventions (skills/, .claude/skills/, plugins/*/skills/, nested dirs).
• Surface the actual SkillError reason in the marketplace (toast + Report bug), not just a silent "failed" pill.

References

• engine/houston-skills/src/remote.rs — install_skill, find_skill_path_in_repo, fetch_skill_md_at_path
• ui/skills/src/community-skills-browser.tsx — handleInstall (swallowed error)
• PR fix(skills): make installed skills appear — YAML-safe frontmatter, preserve + feature, live Windows mirror (#398) #479 / Skill doesn't appear after being installed #398 (where this was flagged)