v0.2.0

Fix: Make token argument required for decode and validate commands (#2)

* feat: add interactive mode with welcome animation

* fix: make token argument required for decode and validate commands

This commit addresses a UX issue where running 'jetta decode' or 'jetta
validate' without arguments would silently wait for stdin input with no
prompt or feedback, confusing users who expected an error message.

Changes:
- Make token parameter required (String instead of Option<String>)
- File flag (--file) takes precedence over token argument
- Explicit stdin reading via '-' argument (e.g., 'echo token | jetta decode -')
- Clap now shows standard error when token is missing
- Update help text to clarify stdin usage with '-'

BREAKING CHANGE: Token argument is now required. To read from stdin,
explicitly use '-' as the token argument instead of relying on implicit
stdin reading when token is omitted.

* test: add integration tests for required token argument

Add comprehensive integration tests to verify the new required token
behavior:

- test_decode_requires_token: Verifies decode shows error when TOKEN missing
- test_validate_requires_token: Verifies validate shows error when TOKEN missing
- test_decode_explicit_stdin: Documents explicit stdin usage with '-' argument
- test_file_takes_precedence: Verifies --file flag takes precedence over token arg

Also adds tempfile dev-dependency for file precedence testing.

Test coverage: 36 tests total (22 unit + 14 integration)

* docs: update README and CHANGELOG for required token argument

Update documentation to reflect the breaking changes in token argument
handling:

README.md:
- Update 'Quick Start' section to show explicit stdin with '-'
- Update 'decode' command documentation with required TOKEN argument
- Update 'validate' command documentation with required TOKEN argument
- Document file precedence behavior
- Add new 'Common Errors' section with troubleshooting for:
  - Missing token argument error
  - Empty stdin error
  - Malformed JWT error

CHANGELOG.md:
- Update [Unreleased] section for v0.2.0
- Document interactive mode and animation features
- Document breaking changes with BREAKING prefix
- Document the UX bug fix
- Note explicit stdin support via '-'

These docs changes complete the breaking change to require explicit
token arguments and stdin handling.

* fix: resolve clippy deprecation warning in tests

Replace deprecated Command::cargo_bin() with assert_cmd::Command::new()
and cargo_bin!() macro to avoid deprecation warnings when running
clippy with -D warnings.

This ensures CI passes with strict clippy checks.

* docs: add mandatory clippy check requirements to AGENTS.md

Add critical quality check instructions to prevent CI breakage.

This is the 3rd time we've broken CI by not running clippy with
-D warnings before committing. The new instructions mandate:

1. Run cargo fmt --all -- --check
2. Run cargo clippy --all-targets --all-features -- -D warnings
3. Run cargo test
4. Only THEN commit

These match our exact CI/CD settings and will prevent future breakage.