docs: adapt contributor guidance and repository policies for Whale

[LunaStev]

Summary

Adapt the contributor-facing documentation and policy structure from Wave to Whale, in one documentation-only commit based on upstream eeb17f1.

• Expand CONTRIBUTING.md with Whale's crate map, fork/branch workflow, signed-off commits, email patch routing, actual default/all-features checks, regression expectations, and review follow-up.
• Explain formatting scope and safe PR updates, including conflict handling, rebase recovery, final diff inspection, and lease-protected pushes.
• Add MAINTAINERS, COPYRIGHT, NOTICE, CODE_OF_CONDUCT.md, and ai.txt.
• Add .github/PULL_REQUEST_TEMPLATE.md with Whale-specific compatibility and validation prompts, and a short contributor navigation section in README.md.

Motivation

Contributors need one entry point that describes this repository rather than Wave's compiler environment. The guide uses Whale's existing CI and component structure; it does not require Wave's LLVM SDK, standard-library policy checks, or maintainer helper scripts that Whale does not have.

Reference: Wave contributor and policy files at ff9f9894.

Target and compatibility impact

No runtime, assembly, object format, IR, CLI, dependency, feature, or MSRV changes.

Whale's existing LICENSE is unchanged. The copyright/notice documents describe Whale's MPL-2.0 repository, without copying Wave's separate standard-library license split. The AI policy retains Wave's complete policy body and 30 language notices, changing only the project heading and update date. The Code of Conduct preserves the existing Contributor Covenant text and attribution, scopes it to Whale, and uses the maintainer email listed in Wave's current MAINTAINERS.

Implementation files, tests, .github/workflows/, .github/ISSUE_TEMPLATE/, and funding configuration are unchanged. No issue-closing directive is included.

Validation

Local documentation checks on the exact contents uploaded through the GitHub connector:

• Verified the change allowlist contains exactly eight documentation/template files.
• Checked 31 relative Markdown link occurrences and all MAINTAINERS file/directory entries against the fetched repository tree and new files.
• Parsed all Markdown fences and ran bash -n on 12 shell examples; this is syntax validation, not execution of the documented commands.
• Checked UTF-8, final newlines, trailing whitespace, conflict markers, and PR checklist structure.
• Reversed the two AI-policy header substitutions and verified the result hashes to the original Wave blob; similarly verified the Code of Conduct source body after reversing its project/contact adaptation.
• Read back the created Git trees and verified the uploaded file blob IDs match the validated local contents. The final compare contains one commit and only the eight intended paths.

Rust build, tests, Clippy, and rustdoc were not run locally because this editing environment has no Rust toolchain. This PR changes prose and a Markdown PR template only; existing CI remains responsible for its normal checks.

Checklist

• The commit includes a DCO Signed-off-by line and preserves attribution.
• No executable behavior changes; documentation validation is described above.
• Contributor-facing guidance and navigation are updated together.
• The final diff contains no unrelated formatting or refactors.
• Existing license text, issue templates, and CI configuration are preserved.