docs: simplify contribution templates

[limityan]

Summary

Simplify the contribution templates introduced by #939:

• Reduce bug, feature, and docs issue forms to the minimum fields needed for triage.
• Replace PR type/area checkbox lists with short free-text fields.
• Collapse PR verification guidance into one concise section and move optional details into reviewer notes.
• Clarify in CONTRIBUTING that contributors should run the smallest relevant verification set, not every row.

Motivation

The first governance PR improved structure, but the templates were still too form-heavy for broader contributors. This keeps useful maintainer context while lowering submission friction.

Verification

• pnpm run check:github-config -> pass
• pnpm run check:repo-hygiene -> pass
• git diff --check -> pass
• git diff --cached --check -> pass

Rust/web/mobile runtime checks were not run because this is a template/documentation-only follow-up.