fix: harden GitHub action entrypoints

[epi052]

Description

Harden the GitHub Action entrypoints used to install and run pickle-fuzzer.

This change removes direct input interpolation in the composite action, pins the
third-party actions used by the Atheris mode, constrains action-managed paths to
GITHUB_WORKSPACE, rejects inverted opcode bounds before invocation, and
replaces same-origin checksum trust with GitHub artifact-attestation
verification for downloaded release binaries.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test addition or update

Changes Made

• Moved inputs.mode validation through an environment variable instead of
  interpolating it directly into a run: block.
• Pinned actions/setup-python and dtolnay/rust-toolchain to immutable SHAs
  for Atheris mode.
• Required explicit version for non-tag action refs, validated release tags,
  and verified downloaded binaries with gh attestation verify against the
  repo release workflow.
• Constrained output_dir, output_file, and Atheris harness paths to stay
  within GITHUB_WORKSPACE.
• Preserved raw args splitting without allowing glob expansion.
• Preserved single-file mode when --mutators is present by inserting --
  before the positional output path.
• Rejected inverted min_opcodes / max_opcodes before invoking the CLI.
• Cleaned the temporary wheel directory, required exactly one built wheel, and
  passed harness_args as a split argv array instead of unquoted shell text.
• Updated action documentation to describe explicit version pinning and
  whitespace-delimited harness_args.

Testing Performed

Describe the tests you ran to verify your changes:

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed
• All existing tests pass (cargo test)

Test commands:

bash -n scripts/action-install.sh scripts/action-run.sh scripts/action-atheris.sh
git diff --check
cargo fmt --check
cargo test

Manual checks:

• scripts/action-install.sh rejects invalid release tags such as ../../evil
• scripts/action-run.sh rejects output_file values that escape
  GITHUB_WORKSPACE
• scripts/action-run.sh keeps positional file arguments separate from repeated
  --mutators
• scripts/action-atheris.sh rejects harness paths outside
  GITHUB_WORKSPACE
• scripts/action-atheris.sh installs exactly one built wheel and passes
  harness_args as a split argv array

Checklist

Before submitting this PR, please ensure:

• Code follows project style guidelines (cargo fmt)
• No clippy warnings (cargo clippy -- -D warnings)
• All tests pass (cargo test)
• Documentation updated (if applicable)
• CHANGELOG.md updated (if applicable)
• No breaking changes (or documented if necessary)
• Commit messages follow conventional commit format

Performance Impact

If this PR affects performance:

• Benchmarks run and results documented
• No significant performance regression

Benchmark results:

N/A