fix: enable OSC 8 hyperlinks by default in TTY terminals

What does this MR do?

Enables OSC 8 terminal hyperlinks by default in TTY output. Previously, hyperlinks were off unless the user opted in with display_hyperlinks: true or FORCE_HYPERLINKS=1.

Changes

• Change the default hyperlink mode from "never" to "auto" in iostreams.New(). "auto" emits hyperlinks only when stdout is a TTY and not paged.
• Centralize the test default to "never" in cmdtest.TestIOStreams so existing tests stay unaffected.
• Add display_hyperlinks: false as the documented opt-out. display_hyperlinks: true now maps to "auto" (preserves the existing TTY-only semantics for users who already opted in).
• Fix a precedence bug where a falsy FORCE_HYPERLINKS=0 or GLAB_FORCE_HYPERLINKS=0 would silently override display_hyperlinks: false. Falsy env values now fall through to the config.
• Extract the hyperlink configuration logic into a testable configureHyperlinks function and add TestMain_configureHyperlinks covering the precedence matrix.
• Add a DisplayHyperlinks() getter to IOStreams for test inspection.

Behavior matrix

Env var	display_hyperlinks	Resulting mode
FORCE_HYPERLINKS=1	any	always
GLAB_FORCE_HYPERLINKS=1	any	always
FORCE_HYPERLINKS=0 or falsy	false	never
FORCE_HYPERLINKS=0 or falsy	true	auto
unset	false	never
unset	true	auto
unset	unset	auto (new default)

How to test

Run glab api --help in a supported terminal without any environment variables or config changes. The "GitLab REST API documentation" and "GitLab GraphQL documentation" links should be clickable.

To verify a falsy FORCE_HYPERLINKS no longer overrides the config opt-out (comment 1 fix):

glab config set display_hyperlinks false
FORCE_HYPERLINKS=0 glab api --help

Links should appear as plain text.

Related issues and merge requests

• feat: render Markdown links as OSC 8 hyperlinks... (!3189 - merged)