D3a [CRIT]: --web mutually exclusive with --json/--jq/--template (C5+C15)

[espadonne]

Summary

C-audit findings C5 + C15: every command that has both --web and the output flags silently picked --web and discarded the machine-readable request. Scripts that pipe --json to jq — common gh-port pattern — got empty stdout and exit 0, no warning.

shithub repo view owner/r --web --json name
# Opening https://shithub.sh/owner/r in your browser.
# exit=0   (--json silently dropped)

shithub issue view 2 -R owner/r --web --json number
# Opening https://shithub.sh/owner/r/issues/2 in your browser.
# exit=0

gh-compat behavior: refuse at parse time with "flag X and Y are mutually exclusive". This PR implements that.

Fix

1. New helper output.MarkWebMutuallyExclusive(cmd) — three pair-wise cmd.MarkFlagsMutuallyExclusive("web", FlagX) declarations so --json + --jq (existing legal combination) still works.
2. Applied to 5 commands: issue/view, issue/list, pr/view, pr/list, repo/view.

Cobra emits a strong message on conflict:

Error: if any flags in the group [web json] are set none of the others can be; [json web] were all set

Test plan

• TestMarkWebMutuallyExclusive — table-driven across all 8 combinations: 4 single-flag cases pass, --json + --jq (legal combo) passes, 3 --web + X cases reject with cobra's mutex message.
• All affected commands' existing tests still green.
• make ci green locally.

Notes for follow-up

repo list --web doesn't exist yet (C23). When it's added in a later D3 PR, the same helper applies — single one-line addition.