Persist installer settings via `apm config` for self-update

[sergio-sisternes-epam]

Summary

PR #660 introduces env vars (APM_INSTALL_DIR, GITHUB_URL, APM_REPO, VERSION) to configure install.sh for air-gapped and GHE environments. These work well for the bootstrap install, but once APM is installed, apm update has no way to remember those settings.

Users who installed to a custom directory or from a GHE mirror would need to re-export env vars every time they run apm update, which is error-prone and defeats the purpose of a self-update command.

Proposed solution

Add persistent config keys via apm config:

Config key	Description
install-dir	Custom binary install directory (default: /usr/local/bin)
mirror-url	GHE or artifactory base URL (default: https://github.com)
mirror-repo	Repository override (default: microsoft/apm)

The flow would be:

1. First install: env vars (only option -- no apm binary yet)
2. Post-install: apm config set mirror-url https://gh.corp.com
3. Self-update: apm update reads config; env vars override if set (CI/unattended)

Acceptance criteria

• apm update reads install-dir, mirror-url, mirror-repo from config
• Env vars override config values when set (for CI/unattended use)
• apm config set mirror-url <url> persists across sessions
• Documentation updated for the new config keys

Related

• feat: make install.sh configurable for air-gapped environments #660 -- introduces the env var configurability for install.sh