feat: add global --jq flag for filtering JSON output

Summary

Adds a per-command --jq <expr> flag for filtering JSON output, implemented with gojq (the same library gh CLI uses).

Stacked on !3294 (merged) — depends on every command's JSON output going through IOStreams.PrintJSON, which !3294 (merged) establishes.

Design

Two registration paths so the flag only appears on commands that emit JSON:

1. cmdutils.EnableJSONOutput(cmd, io, &opts.outputFormat[, desc]) — bundles both --output=text|json and --jq. ~49 commands now use this single helper for both flags.
2. cmdutils.AddJQFlag(cmd, io) — standalone helper for the ~13 commands with bespoke output flag definitions (e.g. variable export with json/env/export, issuable list with --output-format), and the ~4 always-JSON commands (orbit/*) that have no --output flag.

Behavior

• Fail-fast validation: jq expression syntax is checked in PreRunE. If invalid, the command never runs.
• Output mode enforcement: for commands with an --output flag, --jq requires --output=json. The check is in PreRunE, so the command's text output never appears before the error.
• Always-JSON commands: orbit/* commands work directly with --jq since they have no output flag to coordinate.
• Output format: string results emitted raw (matches jq -r); other results emitted as compact JSON, one result per line.
• Error wording: messages lead with a capital word so charm/fang's first-word title-casing doesn't capitalize the --jq flag name (--Jq).

Examples

glab mr list -F json --jq '.[].iid'         # one iid per line, no JSON quoting
glab mr list -F json --jq 'map(select(.draft)) | .[].title'
glab orbit remote tools --jq '.[0]'         # always-JSON command
glab variable list -F json --jq 'length'

# Error paths (all fail before the command runs):
glab mr list --jq '.[]'             # error: requires --output=json
glab mr list -F text --jq '.[]'     # error: explicit conflict
glab mr list -F json --jq '.['      # error: invalid expression
glab version --jq '.foo'            # error: unknown flag (not registered)

Out of scope

glab api --paginate --output ndjson streams documents and does not route through PrintJSON. The existing | jq shell pipeline keeps working for that command. A follow-up MR will add --jq support for the streaming path.

Test plan

• go test ./... passes locally
• golangci-lint run ./... reports 0 issues
• --jq shows in --help only for JSON-emitting commands
• All four error paths fail before any command output appears
• Spot-check glab mr list -F json --jq '.[].iid' against a real project
• Spot-check glab orbit remote tools --jq '.[0]' (always-JSON command)
• Verify glab api projects/... (out of scope) still works with external | jq