improve agent friendliness of view --json

[skarim]

Fix gh stack view --json returning interactive prompt

Problem

When an agent or script runs gh stack view --json and the current branch is either not in a stack or belongs to multiple stacks (e.g. a trunk branch), the command shows an interactive prompt instead of returning valid JSON.

Fix

When --json is set and the stack is ambiguous, return an exit code 2 (branch not in stack) or 6 (branch belongs to multiple stacks).

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[Copilot]

Pull request overview

This PR fixes gh stack view --json producing interactive prompts (which breaks machine-readable output) by routing JSON mode through a non-interactive stack resolution path and returning typed exit codes for ambiguous or missing stacks.

Changes:

• Added a dedicated runViewJSON path that resolves the current stack without invoking interactive disambiguation and returns ErrNotInStack (code 2) / ErrDisambiguate (code 6) as appropriate.
• Adjusted runView control flow so JSON mode exits early (and never shows the interactive loader/prompt flow).
• Added tests covering --json behavior for: branch not in stack, branch in multiple stacks, and branch in a single stack (valid JSON output).

Show a summary per file

File	Description
cmd/view.go	Early-exits JSON mode into a new non-interactive resolver to avoid prompts and provide typed exit codes.
cmd/view_test.go	Adds regression tests validating exit codes and ensuring JSON output remains valid for the single-stack case.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 0