Resolve "[sync] Improve login_enabled field accuracy with native Linux checks"

[geunwoonoh]

Summary

Collect raw login data in alpamon and send to server for improved login_enabled accuracy.

• shadow_expire_date: *int64 - raw value transmitted (days since epoch)
• valid_shells: []string - full /etc/shells list transmitted

Server determines login_enabled using this raw data, enabling policy changes without agent updates.

Changes

pkg/runner/commit_types.go

• Add raw data fields to UserData struct
• Add shadowEntry struct (internal use)
• Update GetData() method

pkg/runner/commit.go

• loadValidShells(): Parse /etc/shells and return []string
• loadShadowData(): Parse /etc/shadow for password lock and expire date
• getUserData(): Collect raw data and include in UserData

pkg/runner/commit_test.go

• TestLoadValidShells: Test /etc/shells parsing
• TestLoadShadowData: Test /etc/shadow parsing
• TestGetUserDataWithRawFields: Test raw data fields

API Changes

Before:

{
  "uid": 4,
  "username": "sync",
  "shell": "/bin/sync"
}

After:

{
  "uid": 4,
  "username": "sync",
  "shell": "/bin/sync",
  "shadow_expire_date": null,
  "valid_shells": ["/bin/sh", "/bin/bash", "/bin/zsh"]
}

Backward Compatibility

alpamon	alpacon-server	Behavior
Old	Old	Server calculates from shell
Old	New	Server calculates from shell (fallback)
New	Old	Server ignores extra fields
New	New	Server uses raw data for accurate determination

Test Plan

• go build ./... succeeds
• go test ./pkg/runner/... passes
• Test /etc/shadow reading on Linux (requires root)
• Integration test with alpacon-server

Related Issues

• Closes [sync] Improve login_enabled field accuracy with native Linux checks #169