Improve project documentation

[rainoftime]

Motivation

• Make the project easier to use and understand by expanding and clarifying the top-level README.
• Document the new/actual CLI entry points and runner modes so users can run and extend fuzzing campaigns without reading source.
• Provide focused guidance for the inter-theory mutation component to help maintainers add bridges and reproduce issues.

Description

• Expand the root README.md with installation instructions (PyPI and source), quick-start examples, command-line summaries, repository layout, development guidance, and bug-reporting instructions.
• Refresh smtfuzz/runner/README.md to document the smtfuzz-runner entry point, seed and gen modes, usage examples, important options, module structure, and extension notes for adding new runners.
• Expand smtfuzz/inter/README.md to list component responsibilities, supported theories, common bridge patterns (e.g. str.len, bv2nat, fp.to_real), usage scenarios, and development notes for adding new bridges.
• Minor formatting and example additions to help reproduce and report solver failures (commands, expected artifacts to save).

Testing

• Ran the test suite with python -m pytest smtfuzz/tests, which executed and reported the existing tests as skipped (7 tests skipped) and completed successfully.
• Verified CLI help commands with python -m smtfuzz.cli --help, python -m smtfuzz.runner_cli --help, python -m smtfuzz.runner_cli seed --help, and python -m smtfuzz.runner_cli gen --help, all of which ran without errors.
• Ran git diff --check (and repository checks) to ensure no trailing whitespace or diff issues; check passed.

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Codex Task