Add devsync update command for installed packages

[troylar]

Description

Add a devsync update command that re-installs packages from their original source with the latest content. This covers both practice file updates and pip-installed MCP server version bumps.

Context

Currently, devsync install is a one-shot operation. If a package source gets updated (new practices, updated MCP configs, version bumps), the only way to get changes is to uninstall and re-install. Teams need a way to pull the latest version of their standards without losing local adaptations.

The package tracker (packages.json) already records the source URL/path, installed tools, and timestamps — enough to support re-fetching.

Affected Files

• devsync/cli/main.py — Register update command
• devsync/cli/update.py — New command implementation
• devsync/storage/package_tracker.py — Query installed packages for source info
• devsync/cli/install_v2.py — Refactor shared logic for reuse by update
• tests/unit/cli/test_update.py — New tests

Acceptance Criteria

• devsync update <package-name> re-fetches and re-installs a specific package
• devsync update --all updates all installed packages
• AI adaptation runs on update (merging new practices with current state)
• --no-ai mode supported for file-copy updates
• Pip MCP packages updated to latest compatible version (when Add pip-installable MCP server support to packages #80 lands)
• Shows diff/summary of what changed before applying
• User confirmation required before overwriting
• Tests pass: invoke test-unit
• Lint passes: invoke lint

Implementation Notes

Reuse install_v2_command internals — update is essentially "re-install from recorded source with conflict=overwrite and changelog output." Consider a --dry-run flag to preview changes without applying.