docs: replace stale RChain-flavored CONTRIBUTING with F1R3FLY.io standard

[jeffrey-l-turner]

Why

The current CONTRIBUTING.md is inherited from RChain and points at dead infrastructure:

• Links to rchain.atlassian.net wiki pages (RChain Atlassian, not F1R3FLY.io)
• References Travis CI, which is no longer used by this repo
• Refers contributors to RChain/README.md rather than this project

This is the file external contributors see first when they consider contributing. Leaving it points new contributors at dead resources and signals the project is unmaintained.

What

Replaces the file with the org-wide standard template proposed in F1R3FLY-io/.github and just landed there (commit 6b71dc9).

Highlights:

• Conventional Commits, branching prefixes, PR review expectations
• Stigmergic-collaboration claim conventions (`docs/ToDos.md`, `docs/work-logs/`)
• Vendor-neutral AI-assist policy: `CLAUDE.md` / `AGENTS.md` / `GEMINI.md` referenced equally; `[agent]` commit prefix; no AI co-author trailers
• Security/PII guidance with reserved-example placeholders
• Apache-2.0 inbound-license statement
• The `BUILD-COMMANDS` block is prefilled with `cargo fmt && cargo clippy && cargo test` for this Rust repo
• `BUILD-COMMANDS:BEGIN/END` and `REPO-SPECIFIC:BEGIN/END` markers exist so a future `/harmonize` sync can refresh the boilerplate without overwriting repo-specific bits

How verified

• The replacement content is byte-identical to the prefilled-Rust variant of the org-wide template
• The two dead-link clusters (RChain Atlassian, Travis CI) are removed
• The Apache-2.0 statement matches `LICENSE`

Notes

This PR also resolves the issue for `F1R3FLY-io/f1r3fly`, which is a rename redirect to this repo (same GitHub repo ID 739209742).

If this repo has Rust-specific contributor steps that go beyond `cargo fmt/clippy/test` (toolchain version, feature flags, integration-test setup, etc.), put them in `DEVELOPER.md` rather than expanding `CONTRIBUTING.md`.