Detect and regenerate shims produced by older generators

[junior-ricon]

Problem

Generated shims can outlive the shiv version and template that produced them.

A concrete example on this machine is mise-managed shiv:codebase@0.3.0. Its active shim still exports generic:

export CALLER_PWD="$PWD"

Current shiv has generated package-scoped variables such as CODEBASE_CALLER_PWD since #113, and the installed codebase source already prefers that variable. Updating shiv or vfox-shiv did not regenerate the existing concrete codebase installation, so the old generated contract persists.

This is adjacent to, but distinct from:

• Rename CALLER_PWD to SHIV_CALLER_PWD across shiv-managed tools #104, which tracks consumer migration away from generic CALLER_PWD;
• Isolate generated shim task maps per mise-managed install #146, which fixes generated shims selecting a shared runtime task-map cache instead of their install-local cache.

Those source fixes do not repair already-generated shims by themselves.

Desired direction

Make stale generated artifacts visible and give each installation model a bounded repair path:

• A traditional global shiv upgrade may regenerate shims owned by its global registry.
• A mise/vfox-managed package rebuild may regenerate only that isolated package installation's shim.
• Neither path should rewrite shims owned by the other model.
• Existing concrete mise package versions need an explicit invalidation/rebuild story when vfox-shiv's bundled shiv generator changes; updating the plugin/bootstrap alone does not currently reinstall them.

Potential mechanisms include generator/format metadata in shim headers, shiv doctor detection, atomic owned-shim regeneration, and a vfox-shiv rebuild contract. The implementation should choose the smallest mechanism that makes ownership and upgrade behavior reliable.

Acceptance criteria

• Generated shims expose enough provenance or format information to detect stale templates.
• Global shiv can atomically regenerate only globally registered shims.
• Mise/vfox-managed installs can rebuild only their version-local shims.
• Upgrading one installation model cannot overwrite the other's shims.
• A regression proves that a package installed with an older shim template is detected and repaired after the relevant generator upgrade.
• Release guidance explains how fixes such as Rename CALLER_PWD to SHIV_CALLER_PWD across shiv-managed tools #104 and Isolate generated shim task maps per mise-managed install #146 propagate to already-installed tools.