chore: listing metadata fixes (teaching category, cursor-sdk description length, agent-compatibility changelog)

[deanrie]

Summary

Three one-line listing metadata fixes across three plugins, consolidated from #525 and #530 (both closed in favour of this one; commits carried unchanged, one each). No behaviour change.

1. fix(teaching): use the productivity category like every other listing (was fix(teaching): use the productivity category like every other listing #525). teaching is the only plugin whose plugin.json says "category": "utilities"; the other listings use exactly integrations, developer-tools or productivity. The schema does not constrain the field, so the validator is silent, but teaching is also one of the entries with no page on cursor.com/marketplace (Can't find the teaching plugin on the Cursor Marketplace #364) and an unrecognised category is the cheapest thing to rule out. Moved to productivity, README row updated to match.
2. chore: keep cursor-sdk skill description under the 1024-char limit; label agent-compatibility changelog as 1.0.0 (was chore: keep the cursor-sdk skill description under the 1024-char limit; label agent-compatibility changelog 1.0.0 #530). cursor-sdk/skills/cursor-sdk/SKILL.md had a 1059-character description; skill descriptions are cut at about 1024, so the tail was lost in the picker and in what the model sees. Trimmed to 982 characters, no trigger phrases dropped. agent-compatibility/CHANGELOG.md had only ## Unreleased while plugin.json ships 1.0.0; heading renamed to ## 1.0.0.

Verification

• node scripts/validate-plugins.mjs: All plugins validated successfully. (with ci: pin validator deps, check SKILL.md frontmatter, run script-package tests #618 the cursor-sdk description warning goes away on this branch.)

Refs #364.