docs: update BSTVC homepage links and companion tools

CODE_OF_CONDUCT.md

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+# BSTVC Code of Conduct
+
+BSTVC is committed to a respectful, inclusive, and scientifically constructive
+community.
+
+## Expected conduct
+
+Participants are expected to:
+
+- communicate respectfully and focus criticism on ideas, evidence, and code;
+- welcome questions and contributions from people with different backgrounds
+  and levels of experience;
+- acknowledge uncertainty, methodological limitations, and the contributions
+  of others;
+- protect confidential, personal, patient, student, and restricted data; and
+- avoid harassment, discrimination, intimidation, personal attacks, and other
+  disruptive behavior.
+
+## Scope
+
+This code applies to project repositories, issue discussions, pull requests,
+documentation, and other spaces in which a person represents the BSTVC
+community.
+
+## Reporting
+
+Potential violations may be reported privately to the project maintainer,
+[Chao Song](mailto:chaosong.gis@gmail.com). Reports will be reviewed as
+confidentially and fairly as reasonably possible. Project maintainers may
+edit or remove inappropriate contributions and may temporarily or permanently
+restrict participation when necessary to protect the community.

CONTRIBUTING.md

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+# Contributing to BSTVC
+
+Thank you for helping improve BSTVC. Contributions may include reproducible
+bug reports, documentation corrections, worked examples, tests, scientific
+validation, or code changes.
+
+## Before contributing
+
+1. Search the [issue tracker](https://github.com/bayesianstvc/BSTVC/issues)
+   for an existing report or proposal.
+2. Open an issue before making a substantial change so that its scientific
+   scope, expected behavior, and compatibility implications can be discussed.
+3. Do not include confidential, identifiable, proprietary, or otherwise
+   restricted data in issues, examples, tests, or pull requests.
+
+## Bug reports
+
+A useful bug report should include:
+
+- the smallest reproducible example;
+- the expected and observed behavior;
+- the BSTVC, R, INLA, and operating-system versions;
+- relevant warnings or error messages; and
+- synthetic or openly shareable data whenever data are required.
+
+## Code and documentation contributions
+
+- Keep each pull request focused on one clearly described change.
+- Preserve backward compatibility unless a breaking change has been discussed.
+- Add or update documentation and examples when user-facing behavior changes.
+- Add suitable tests for corrected or newly introduced behavior.
+- Describe the scientific rationale for changes that affect model
+  specification, inference, diagnostics, or interpretation.
+- Confirm that examples and checks complete successfully before submission.
+
+By participating in this project, contributors agree to follow the
+[Code of Conduct](./CODE_OF_CONDUCT.md).