Bump internal action refs to 2.6.0

[ssestak]

Summary

Prepares the 2.6.0 release by pointing every internal futuredapp/.github/... action and workflow reference from @2.5.0 to @2.6.0, via .github/scripts/bump-action-refs.sh 2.6.0.

Changes

• 15 reusable workflows and the ios-kmp-build composite action updated (38 refs)
• No functional changes besides the version refs
• validate-action-refs.sh 2.6.0 and actionlint pass

Release contents

Already on main:

• android-cloud-check: SETUP_RUBY input to skip setup-ruby on runners that ship Ruby #124 android-cloud-check: SETUP_RUBY input, fixes RUNS_ON: self-hosted dying in setup-ruby on the mac runners
• Cache Kotlin/Native library caches in iOS KMP PR checks #125 ios-kmp-selfhosted-test: kotlin_native_cache input (on by default), caches Kotlin/Native library caches between PR checks
• CLAUDE.md: public repository conventions

Pending:

• android: BUILD_RUNS_ON, in-job Android SDK, Ruby and Kotlin/Native cache for build jobs #127 BUILD_RUNS_ON + in-job Android SDK / Ruby / Kotlin/Native cache inputs for nightly, Firebase release, Google Play release and KMP combined nightly build jobs; CLAUDE.md release docs

Merge order

1. android: BUILD_RUNS_ON, in-job Android SDK, Ruby and Kotlin/Native cache for build jobs #127
2. this PR, then create tag 2.6.0 on the merge commit (validate-release-tag.yml checks the refs)

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