Add structured GitHub issue templates

[huazhuang80-star]

What & why

Adds GitHub issue forms so bug reports and feature requests arrive with enough context, and routes security reports to the private advisory flow described in SECURITY.md.

Type of change

• docs
• chore / ci

Summary

• Add a bug report form with reproduction steps, affected area, OS, Node/Rust/Stellar CLI versions, and logs.
• Add a feature request form with problem, proposal, affected area, alternatives, impact, and references.
• Add issue template config that disables blank issues and links to private security advisories, SECURITY.md, and the contributing guide.

Validation

• Verified the new files are present on the fork via the GitHub contents API.
• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --locked
• stellar contract build
• Frontend: npm run lint, npx tsc -b --noEmit, npm run build, npx vitest run
• npm run verify:deployment
• Not run; GitHub issue-template-only change.

Closes #65