Add `--watch` flag to re-scan on file changes

[lonerthefirst3-sudo]

Summary

Add a --watch flag to the scan subcommand that keeps the process alive and re-runs the scan whenever a .rs file under the scanned path is modified.

Proposed UX

soroban-guard scan ./src --watch

The process should:

1. Run an initial scan and print results as normal.
2. Watch for file-system events using the notify crate.
3. On any .rs file change under the path, re-run the full scan and reprint results (with a timestamp header).
4. Exit on Ctrl-C.

Implementation hints

• Add notify to crates/cli/Cargo.toml as an optional or default dependency
• Gate the watch loop behind the --watch flag in Commands::Scan
• Clear the terminal before each re-scan (optional, but improves UX)
• Respect existing --exclude patterns during watch re-scans

[Mistersmile4585]

@Mistersmile4585 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintenaner, I’m a full stack developer with experience in smart contract development and modern web technologies. I’m comfortable navigating existing codebases, understanding project requirements, and following contribution guidelines. I will be glad if this issue is assigned to me.

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[alade-dev]

@alade-dev has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

please I would love to take on this issue. Thanks

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[rahimatonize]

@rahimatonize has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi maintainer, i would love to work on this issue, please assign

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[starkiVector-090]

@starkiVector-090 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, I'll love to work on this issue

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