Open transcript URLs on modifier click

[TheGreatAxios]

Summary

• Holding the platform modifier over an http(s) URL in a transcript row underlines it, and releasing a press on the same URL opens it in the default browser
• Plain clicks, drag selection, mouse-capture-off behavior, and non-http(s) targets behave exactly as before
• Markdown prose stays terminal-owned, since its renderers expose no text-leaf API to highlight or hit-test

Verification

• bun run typecheck, bun run build, and bun run check pass
• New unit and transcript integration tests cover scheme gating, hit-testing, hover highlight, open-on-release, and unchanged plain-click behavior
• Modifier reporting depends on the terminal sending key-modifier mouse events and needs a real-terminal check

Related to CL-7346

[linear-code]

CL-7346

[TheGreatAxios]

Audit: NEEDS-WORK (security — Windows opener command injection)

Finding (blocking): defaultUrlOpener (src/tui/url-links.ts) routes Windows opens through ["cmd", "/c", "start", "", url]. Bun.spawn with an argv array does not invoke a shell itself, but cmd.exe /c re-parses the assembled command line, so &, | and && inside a transcript URL act as command separators. Transcript URLs are attacker-influenceable (tool output, fetched pages, pasted logs), so Ctrl+clicking a crafted URL on Windows would execute arbitrary commands. Fix: never route through cmd /c — spawn rundll32 url.dll,FileProtocolHandler <url> as argv with no shell. I will push this fix to this branch.

Checked and kept (no change requested):

• Scheme gate parses (new URL(url).protocol) rather than prefix-matching, so javascript: / file: / mailto: never open.
• argv-spawn with no shell on the darwin/linux paths.
• Press/release URL equality: a press that releases anywhere else stays a selection gesture.
• Stale-handler disarm on retext, uppercase-scheme matching, button-0-only (right-click safe) gating.

Suggestion (non-blocking): a hover URL preview (status line or tooltip showing the target) would mitigate display-text-vs-target phishing; consider as a follow-up.

Verification note: the unit/headless suites pin the gating with a mocked opener, but a real-terminal pass is still needed to confirm the terminal delivers the held modifier on motion/press events and that the spawn resolves into a browser. Not run in this pass (no real-terminal mouse tests here).