ref(skills): move superseded skills to skills-legacy

[evanpurkhiser]

Per the decision to retire the per-SDK skills in favor of the sentry-instrument skill and the references/sdks/ trees, this moves the superseded skills under skills-legacy/. Living there keeps them out of the plugin build (which reads skills/) and out of the nightly drift automation, while skills.sentry.dev can still serve them as a fallback so existing /sentry-*-sdk/SKILL.md URLs keep resolving.

Moves 26 skills: the 19 per-SDK skills plus sentry-sdk-skill-creator, sentry-fix-issues, the three sentry-span-streaming-* skills, sentry-instrument-logging, and sentry-instrumentation-guide. Regenerates SKILL_TREE.md and repaths the moved entries in .github/CODEOWNERS.

sentry-sdk is intentionally left in skills/ — its removal is handled in a separate PR.

Sequencing note: because the SDK skills are gone from skills/, the regenerated SKILL_TREE.md now has an empty "SDK Setup" table and empty "Quick Lookup", and the sentry-sdk-setup router still links to the moved skills. That is expected — this should land together with (or just before) the skills-next → root merge, which replaces the index and routers wholesale with sentry-get-started / sentry-instrument. It should not be deployed standalone. The skills.sentry.dev skills-legacy/ fallback and the CODEOWNERS ownership for references/sdks/ are deliberately out of scope here and land separately.

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[sentry]

Bug: Agent definition files in .github/agents/ contain hardcoded paths to skills that were moved to skills-legacy/. The agents will fail to load their knowledge base.
_{Severity: MEDIUM}

Suggested Fix

Update all hardcoded file paths in .github/agents/skill-updater.agent.md and .github/agents/skill-creator.agent.md. Specifically, change references from skills/sentry-sdk-skill-creator/, skills/sentry-go-sdk/, and skills/sentry-nextjs-sdk/ to their corresponding skills-legacy/ paths.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: SKILL_TREE.md#L60-L72

Potential issue: The agent definition files `.github/agents/skill-updater.agent.md` and
`.github/agents/skill-creator.agent.md` contain hardcoded file paths referencing skills
in the `skills/` directory. However, this pull request moves those skills to the
`skills-legacy/` directory. When the `skill-updater` or `skill-creator` agents are
invoked, they will attempt to read their knowledge base from these outdated paths. The
read operations will fail, preventing the agents from accessing their source material
and causing them to produce incorrect or nonsensical outputs.

Also affects:

• .github/agents/skill-updater.agent.md:30~43
• .github/agents/skill-creator.agent.md:31~44

_{Did we get this right? 👍 / 👎 to inform future reviews.}