Fix deprecated Homebrew macOS dependency syntax

[r3dbars]

Why

Issue #1707 reports Homebrew's deprecation warning for the cask's string-form macOS dependency.

Product Impact

• Affects: docs only / release metadata
• Lane: release ops
• Why this matters: Homebrew can load the cask without the deprecated string-comparison warning while keeping the macOS 26+ install floor.

What changed

• Changed depends_on macos: ">= :tahoe" to Homebrew's supported symbol form depends_on macos: :tahoe.
• Updated the release metadata contract test to protect the supported syntax and existing macOS floor.

How I checked it

• scripts/dev/agent-preflight.sh
• Selected checks from .agents/test-matrix.yml for the files changed
• bash build-deps.sh --force
• bash build.sh --no-open
• bash run-tests.sh — 12,622 passed
• Targeted bash run-tests.sh --filter ReleaseMetadataContractTests — 28 passed
• SKIP_NOTARIZATION=1 bash build-beta.sh '' codex — local package smoke passed; no publish or notarization
• ruby -c Casks/transcripted.rb and git diff --check
• Manual Homebrew load/audit — Homebrew CLI is not installed in this environment

Risk Review

• Privacy / local-first behavior reviewed
• Storage path or migration impact reviewed — none
• Public-facing copy stays concrete and matches current product scope
• Release/update impact reviewed — only the cask DSL and its contract assertion changed; Sparkle, appcast, Sentry, download, and release artifacts were not changed
• Agent PR links the issue and stays draft until human review
• No private transcripts, audio, tokens, personal paths, or customer data are included

Notes

• Current upstream source is r3dbars/transcripted, default branch main; no separate r3dbars/homebrew-transcripted repository was needed.
• Independent codex review --base origin/main found no issues and confirmed that the symbol-form change preserves the macOS 26+ minimum.
• This PR does not merge, close the issue, publish a release, update Sparkle/appcast, or change the DMG/checksum.

Agent handoff

COORD_DONE: GREEN | PR to be added after creation | cask uses Homebrew symbol-form macOS dependency and its release contract test is updated | leave issue #1707 open; human review/merge only | no decisions needed | deps build, app build, 12,622 fast tests, 28 targeted contract tests, package smoke, Ruby syntax, diff check, independent review | smallest next action: review and merge the draft PR when appropriate