ci: fall back to published interscript-maps gem when ../maps is absent

[ronaldtse]

Summary

• CI rake workflow failed on every bundle command: the Gemfile hardcoded interscript-maps as a path dependency on ../maps, which is never checked out in CI (exit 13, 'The path ../maps does not exist')
• fall back to the published RubyGems interscript-maps gem when the sibling checkout is absent — the same pattern the secryst group already uses

What this fixes vs what remains

• FIXED: bundling in CI (all jobs now install and run)
• REMAINING (pre-existing, out of scope here): 23 standardrb lint offenses on main; pycall-based specs fail in CI (PyCall.import_module("regex") — python engine unavailable on the runners)

Test plan

• bundle/lock succeeds in every matrix job
• full rake green (needs the pycall + lint debt addressed separately)