Add --version flag to aegisflow binary

[saivedant169]

Problem

Running ./bin/aegisflow --version currently prints nothing useful. The version constant lives inside cmd/aegisflow/main.go but there is no CLI flag to surface it.

Users need a way to check what version they are running without grepping logs or reading the binary.

Acceptance criteria

• Adding --version to the aegisflow command prints the version string and exits 0
• Adding -v as a shorthand also works
• Output format: aegisflow v0.5.0
• Works before any config file is loaded (no config required)
• A unit test covers it in cmd/aegisflow/main_test.go (or a new file if one does not exist)

Files to touch

• cmd/aegisflow/main.go (the version constant is already there)
• A new test file if needed

How to test locally

make build
./bin/aegisflow --version
# Expected: aegisflow v0.5.0

./bin/aegisflow -v
# Expected: aegisflow v0.5.0

Notes

Keep it simple. No cobra/cli library unless we already use one. Use the standard flag package.