cli: add update check + self-update command

[timbrinded]

Summary

Add first-class update awareness to hlgo:

1. Detect when a newer release exists.
2. Provide a CLI command to update the binary in place.

Problem

Today users must manually discover and install new versions. That increases drift and slows rollout of fixes.

Goals

• Expose a machine-readable way to know whether an update is available.
• Provide an explicit command to update to the latest release (or a specific version).
• Keep behavior safe, deterministic, and scriptable.

Proposed UX

• hlgo version --check (or hlgo update --check) returns JSON with:
  • current_version
  • latest_version
  • update_available (bool)
• hlgo update installs the latest compatible release for current OS/arch.
• hlgo update --version vX.Y.Z installs a pinned version.
• hlgo update --dry-run prints planned artifact URL + target install path without modifying filesystem.

Requirements

• No auto-update side effects on normal command execution.
• Structured JSON output and stable error codes.
• Verify checksums/signatures of release artifacts before replacing binary.
• Atomic replace strategy (download -> verify -> swap) with rollback on failure.
• Respect install location overrides when possible.

Non-goals

• Background updater daemon.
• Silent automatic updates during trading commands.

Acceptance Criteria

• Update check endpoint integration (GitHub Releases or equivalent) with retry/error handling.
• hlgo update command implemented with latest + pinned version support.
• Dry-run mode for update command.
• Integrity verification before install.
• Cross-platform behavior documented (Linux/macOS at minimum).
• Unit tests for version parsing, channel selection, and failure modes.
• E2E test for successful update flow in CI-safe mocked environment.