feat: add auto-upgrade system with /upgrade command

[dayanstef]

Add automatic version checking and upgrade capability:

• Check for updates on startup (async, non-blocking)
• Detect install method (Homebrew, go install, curl script)
• /upgrade command with subcommands (check, now, skip, changelog, status)
• Skip versions user doesn't want to install
• Configurable via upgrade.auto_check in config

New package: internal/upgrade/

• types.go: ReleaseInfo, UpgradeState, CheckResult
• checker.go: GitHub API version checking
• installer.go: Binary download and installation
• checker_test.go: Unit tests for version comparison

Description

Brief description of what this PR does.

Related Issue

Fixes #(issue number) or Related to #(issue number)

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

How Has This Been Tested?

Describe the tests you ran:

• Tested locally with Ollama
• Added/updated unit tests
• Tested on macOS
• Tested on Linux

Checklist

• My code follows the project style guidelines
• I have performed a self-review of my code
• gofmt -s -l . shows no files (code is formatted)
• go vet ./... passes
• make test passes
• make build succeeds
• I have updated documentation if needed
• My changes generate no new warnings