gh integration: retire the `current` symlink once consumers resolve through `dev deps path gh`

[JPDuchesne]

Context

GhIntegration#publish_current maintains <install_dir>/current -> <tag> so consumers that cannot ask dev can hardcode one path. #176 adds dev deps path gh <name>, which resolves the version-keyed dir from the lockfile through DataRoot — the answer current was standing in for, minus the two things wrong with it:

• It is machine state ("last installed"), not a dependency declaration; two checkouts with different locked tags share one pointer.
• It is a write on the shared engine tree. The read-only ai-agent identity crashed on it (plans#26, caught live via dev install-deps) — the skip-if-converged guard in publish_current exists only to dodge that. The accessor is pure read.

Blocked on

cellbound-3d migrating its consumers (all six):

• tooling/editor/lib/editor/editor_host.rb DEFAULT_ENGINE_DIR
• bin/ue-bake DEFAULT_ENGINE_DIR
• bin/build-game.sh, bin/test-game.sh, bin/runuat.sh, bin/build-sim.sh — $ue_root/current fallbacks (dead in the container lane: BuildContainer#resolve_versioned_volumes already mounts the version-keyed dir at /ue)

No other gh-dep consumer found in the org (unreal-engine uses deps path xcode only; satisfactory-server is steam).

Done when

• publish_current and its call sites in install_prebuilt / install_from_source are deleted, with tests.
• The gh integration's class comment no longer describes the pointer.
• Existing current links on hosts are left alone (dangling-safe: nothing reads them) — or dev up removes them once, decide in the PR.