docs: add issue and pull request templates

[umeraamir09]

Summary

• Add concise GitHub issue forms for bug reports and feature requests, using the existing bug and enhancement labels.
• Add issue-template configuration that keeps the blank issue option available.
• Add a pull request template for the problem, user/configuration impact, validation, documentation, secret hygiene, and optional UI evidence.

Validation

• Parsed both issue forms and the issue configuration as YAML; checked supported field types, unique IDs, labels, and required-field flags.
• Checked the PR template sections and checklist.
• git diff --cached --check passed.

Relationship to #63 and #64

This PR is intentionally based on docs/open-source-readiness (PR #63), so the diff contains only the templates while aligning with the contribution guidance and checks introduced there. PR #64 is separately stacked on #63; this PR does not include or depend on its README changes. Retarget this PR to main after #63 merges.

[github-actions]

Model not found: opencode/deepseek-v4-flash-free. Did you mean: ling-3.0-flash-fin-free, ling-3.1-flash-free, mimo-v2.6-flash-free?

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