chore(repo): add flutter-version-bump skill for v9

[VelikovPetar]

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Linear: FLU-705

Github Issue: #

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• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

Ports the flutter-version-bump skill from master to the v9 branch, adapted to v9's own infrastructure instead of a blind copy:

• Floor policy: v9's minimum (Flutter 3.27.4 / Dart 3.6.2) is frozen, not "latest stable − 1" — a floor raise is framed as an exceptional maintainer decision, including the repo-wide tall-style reformat that crossing Dart 3.7 would trigger.
• CI facts corrected for v9: the "3.x" auto-adopt pin lives in stream_flutter_workflow.yml's env (plus update_goldens.yml and the pana action) rather than a shared setup-flutter action; update_goldens.yml takes no target input; package_analysis runs the non-golden test suite on the floor, not just lib/ analysis; the melos.yaml shared dependency catalog must move together with package pubspecs on dependency floor bumps (precedent: chore(repo): support Flutter 3.47 #2895).
• v9-specific traps: stale .dart_tool/package_config.json flipping dart format to the tall style; the committed seven-entry analyzer.exclude block; minimal error-floor Gradle/Kotlin bumps as the branch convention.

The skill was verified by replaying the Flutter 3.47 adoption: a throwaway worktree at the commit before #2895, with 3.27.4 and 3.47.0 side by side, executing every locally-runnable step and scoring output against #2895's actual diff. The replay rediscovered everything mechanically discoverable in that PR (golden drift, the two Gradle/Kotlin floor violations, the analyzer.exclude injection) and caught five defects that are fixed in this version: a broken failure-set normalization in the test A/B, a stale-lockfile blind spot (alchemist 0.11.0 fails to compile on 3.47, masquerading as 100+ test regressions), a misstated Android precedent, undocumented macOS attribution distortions, and the beta canary running against the default branch only.

Test instructions: none — documentation-only change, not exercised by CI.

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No UI changes.

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