docs(cli): examples + CHANGELOG + CONTRIBUTING

[sirambrosio]

Summary

DX polish — three complementary pieces making bspctl easier to adopt, contribute to, and version.

• `examples/` — runnable bash scripts for the three core flows (create BEO, grant consent, submit BioRecord). Each script includes argument validation, inline docs, and next-step hints.
• `CHANGELOG.md` — Keep a Changelog format, SemVer, covers 0.1.0 -> 0.2.0 history plus [Unreleased] section.
• `CONTRIBUTING.md` — dev setup, PR checklist, coding style, conventional commits, security rules around private keys, release process.
• `README.md` — expanded "Command Reference - Detailed" section covering all 22 commands with every flag (required / optional / default) and example invocations.

Test plan

• `bash examples/create-beo.sh alice-test.bsp` walks through creation on testnet
• `bash examples/grant-consent.sh ` issues a token with default scope
• `bash examples/submit-biorecord.sh fixtures/sample.json` submits successfully
• All internal README links resolve
• Link to `bsp-spec/docs/ERROR_CODES.md` resolves once that PR merges

🤖 Generated with Claude Code