agr sync --upgrade: re-resolve dependencies to latest

[kasperjunge]

Summary

Add an --upgrade flag to agr sync that re-resolves all dependencies to their latest upstream versions and updates agr.lock.

Why

Many users coming from npm/pip/cargo expect an explicit "update" command. While uv omits this deliberately, the discoverability gap confuses users unfamiliar with the uv model. agr sync --upgrade provides a clear verb for "bump my dependencies to latest."

Scope

• Add --upgrade flag to agr sync
• When set: re-resolve all dependencies to latest HEAD (ignoring lockfile pins), update agr.lock, install
• Without flag: agr sync continues to install from lockfile as usual
• Consider also --upgrade-skill <handle> for upgrading a single dependency
• Depends on: lockfile (Lockfile: add agr.lock for reproducible installs #497)

Design context

This follows the pattern of uv lock --upgrade / cargo update. We chose not to add a separate agr update subcommand to keep the CLI surface small, but --upgrade on sync achieves the same goal.

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