fix(update): make /gsd:update runtime-aware and target correct runtime

[obouafif]

Problem

When multiple runtimes are installed (for example Claude + Codex), /gsd:update could target the wrong runtime because the update workflow did not explicitly carry runtime selection through to install execution.

In practice, Codex users could end up with Claude-targeted update behavior instead of a Codex-targeted update path.

What this changes

• Adds runtime-aware detection in the update workflow (claude, opencode, gemini, codex).
• Introduces PREFERRED_RUNTIME (derived from execution context, with env-var fallback).
• Reorders runtime probing so the preferred runtime is checked first.
• Returns and parses 3 values from detection step:
  • installed version
  • install scope (LOCAL/GLOBAL/UNKNOWN)
  • target runtime
• Executes update with explicit runtime flag:
  • npx -y get-shit-done-cc@latest "--$TARGET_RUNTIME" --local|--global
• Keeps safe fallback for unknown installs:
  • --claude --global
• Updates user-facing text to runtime-neutral wording and includes codex paths in cache-clear/install path notes.

Why this is correct

The installer already supports explicit runtime flags (--claude, --opencode, --gemini, --codex). This change ensures /gsd:update passes the right runtime explicitly instead of relying on default behavior.

Validation

• Ran full test suite: npm test
• Result: 527 passed, 0 failed