fix(app): default review tab to the branch diff on feature branches

[stroopc-owner]

Problem

The Forge review tab showed the wrong git changes for a session on a pushed feature branch (reported: Forge task tsk-LlkvyXEfseuI, related to owner/Coder#614). The tab surfaced only an unrelated .opencode/package.json version bump, not the task's actual committed work.

Root cause: the review-tab change mode defaulted to git (working tree vs HEAD = uncommitted changes only) in packages/app/src/pages/session.tsx. After a Forge task commits and pushes, its real changes live in HEAD, so git mode shows nothing from them — only leftover working-tree drift.

That drift is itself expected: on startup opencode runs Npm.install(dir, { add: [{ name: '@opencode-ai/plugin', version: InstallationVersion }] }) for each .opencode dir (packages/opencode/src/config/config.ts). InstallationVersion is the running CLI version (1.17.11), so npm rewrites the monorepo's committed .opencode/package.json from ^1.14.18 to the pinned resolved version — which is what appeared as the "only" change.

Fix

Prefer the branch diff (current branch vs default branch = the session's committed work) as the auto-default review mode when a branch option is available, instead of git. An explicit user selection is preserved (new changesTouched flag) and only overridden when the chosen mode is no longer available.

Pure ordering/resolution logic is extracted to reviewChangeOptions / resolveReviewChangeMode in session/helpers.ts so it is unit-testable.

Tests

• packages/app/src/pages/session/helpers.test.ts: 11 new cases covering option ordering (feature branch, default branch, missing metadata, no-git) and default resolution (auto-select branch, preserve explicit choice, fall back when unavailable, empty options).
• bun test src/pages/session/helpers.test.ts → 21 pass / 0 fail.
• bun typecheck → exit 0. oxlint on changed files → 0 errors.

Note (not fixed here)

The .opencode/package.json startup rewrite lives in the owner monorepo tooling; pinning @opencode-ai/plugin there would stop the drift at its source. Filed separately — this PR only fixes the review-tab base so the correct diff shows regardless.

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