fix: gh pr/issue/run view drops flags when no identifier given

[shadowofdoom]

Problem

view_pr(), view_issue(), and view_run() all assume args[0] is an identifier (PR number, issue number, run ID). But gh allows calling view without an identifier — it defaults to the current branch.

When users run gh pr view --json number,title,... (no PR number), the rtk-rewrite hook transforms it to rtk gh pr view --json number,title,.... Then view_pr() takes "--json" as the PR number and appends its own --json fields:

gh pr view --json --json number,title,state,author,body,...
           ^^^^^^
           treated as PR number, then rtk adds its own --json

Result: Unknown JSON field: "--json" — gh interprets the double --json as a field name.

Same pattern breaks:

• gh pr view --json fields → Unknown JSON field: "--json"
• gh issue view --json fields → same error
• gh pr view 42 --json custom,fields → rtk's hardcoded fields override user's

This is a generalization of the view_run fix from PR #159, which only handled --log-failed/--log/--json after a run ID but not the no-identifier case.

Fix

Replace the narrow should_passthrough_run_view() with a generic should_passthrough_gh_view() that detects:

1. First arg is a flag (--json, --web, etc.) — no identifier given, gh will default to current branch
2. Any arg is --json, --log, or --log-failed — output incompatible with rtk's summary filter

Applied to all three functions: view_pr(), view_issue(), view_run().

Also simplifies the passthrough plumbing: removes run_passthrough_with_extra() in favor of reusing the existing run_passthrough() with a small prepend_arg() helper.

Test plan

• 7 unit tests for should_passthrough_gh_view():
  • --json as first arg (no identifier) → passthrough
  • --json after identifier → passthrough
  • --log-failed after identifier → passthrough
  • --log after identifier → passthrough
  • --web as first arg (no identifier) → passthrough
  • identifier only → no passthrough (use filter)
  • empty args → no passthrough
• All 414 tests pass (cargo test)
• cargo fmt and cargo clippy clean
• Manual: rtk gh pr view --json number,title returns JSON (no error)
• Manual: rtk gh pr view 42 --json number,title returns user-requested JSON

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