Add issue template, PR template, and CONTRIBUTING.md

- Issue template requests full SQL query and suggests EXPLAIN AST output
- PR template notes that external PRs are not accepted
- CONTRIBUTING.md explains that all code is written by Claude

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kyleconroy

.github/ISSUE_TEMPLATE/parser_bug.md

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+---
+name: Parser Bug
+about: Report a SQL query that doesn't parse correctly
+title: ''
+labels: bug
+assignees: ''
+---
+
+## SQL Query
+
+Please provide the **complete** SQL query that fails to parse:
+
+```sql
+-- Paste your SQL query here
+```
+
+## Expected Behavior
+
+Describe what you expected to happen.
+
+## Actual Behavior
+
+Describe what actually happened (error message, incorrect AST, etc.).
+
+## EXPLAIN AST Output (Optional but Helpful)
+
+If possible, provide the ClickHouse `EXPLAIN AST` output for comparison:
+
+```
+-- Run: clickhouse client --query "EXPLAIN AST <your query>"
+-- Paste the output here
+```
+
+## Additional Context
+
+- ClickHouse version (if relevant):
+- Any other context about the problem:

.github/PULL_REQUEST_TEMPLATE.md

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+## Important
+
+This repository does not accept pull requests. All code is written by Claude (AI).
+
+If you've found a bug or want to request a feature, please [open an issue](https://github.com/sqlc-dev/doubleclick/issues/new) instead.
+
+---
+
+If you're a maintainer, please provide:
+
+## Summary
+
+## Test Plan

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in doubleclick!
+
+## Pull Requests
+
+**This repository does not accept pull requests.** All code is written by [Claude](https://claude.ai), an AI assistant by Anthropic.
+
+## Reporting Bugs
+
+If you've found a bug, please [open an issue](https://github.com/sqlc-dev/doubleclick/issues/new?template=parser_bug.md) with:
+
+1. The **complete SQL query** that fails to parse
+2. The expected behavior
+3. The actual behavior (error message or incorrect output)
+4. Optionally, the ClickHouse `EXPLAIN AST` output for comparison
+
+## Feature Requests
+
+For feature requests, please [open an issue](https://github.com/sqlc-dev/doubleclick/issues/new) describing the functionality you'd like to see.