Add file-based token storage for Linux to avoid dbus dependency

[Copilot]

Implements the same keyring abstraction pattern from jira-cli PR #19. The go-keyring library requires dbus on Linux, which may not be available in containerized or minimal environments.

Changes

• New internal/keyring package with platform-specific implementations via build tags
  • Linux: File-based storage at ~/.config/slack-cli/token (JSON format, 0600 permissions)
  • macOS/Windows: System keyring via go-keyring (unchanged behavior)
• Updated main.go to use internal/keyring instead of direct go-keyring import

Example

On Linux, tokens are now stored as JSON instead of requiring dbus:

{
  "SLACK_TOKEN": "xoxb-..."
}

The API remains identical - slack configure and token retrieval work the same across all platforms.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• slack.com
  • Triggering command: /tmp/go-build45997241/b001/slack-cli.test -test.testlogfile=/tmp/go-build45997241/b001/testlog.txt -test.paniconexit0 -test.timeout=10m0s (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

Look at kitproj/jira-cli#19 and do the same here.

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