feat(web): one Linked items panel for pull requests and issues

[TonybynMp4]

Follow-up to pingdotgg#6315, as offered there. It targets feat/issues-page and holds a single commit.

Problem

Linked pull requests have a right-panel tab and a Link dialog. Linked issues only appear in the header popover, and nothing in the panel can link one.

Change

• The thread's Pull requests tab becomes Linked items and lists linked issues under its pull requests. The panel shows whenever the thread has either kind, and the header's issue count opens it.
• One Link dialog (LinkThreadItemDialog) replaces LinkPullRequestDialog and links issues too. It detects the kind from a pasted URL and offers a toggle for bare numbers. A pasted URL the thread already has is refused up front.
• In the command palette, Show linked items works in issue-only environments, and Link issue to thread opens the dialog on issues.
• Rows use three lines, since the tab is a narrow side panel: the reference with the diff counts and time, then the title with the watch, checks and review icons, then the stack, author and branches. Issue rows share the layout. They show the reference and title, plus the same … menu as pull requests: Copy link, Open, Unlink from thread.
• Unlinking an issue from the panel reports failures.

The row layout lives in LinkedItemRow.tsx, shared by both kinds. The PR reference parsing moved out of the old dialog into linkPullRequestReference.ts, so both kinds share one dialog.

This branch also carries issueReference.ts and useIssueLinking from the chat link context menu PR, so it can merge on its own. Whichever of the two lands second sees them as unchanged.

Screenshots

Command palette: Link pull request / Link issue / Show linked items

Linked items panel with a PR and two issues

Link dialog with pingdotgg#15677 on the Issue toggle

Pasted URL that is already linked

Verification

• vp test run apps/web/src/components/LinkThreadItemDialog.logic.test.ts apps/web/src/components/chat/ThreadIssueLinks.test.tsx passes 8/8.
• tsc --noEmit in apps/web and lint on the changed files are clean.
• Checked by hand in a dev build: linked [Bug]: Subagent thread receives the literal prompt PROMPT_PLACEHOLDER instead of its task pingdotgg/t3code#15677 from the palette dialog, saw it appear in the panel, and pasting its URL again was refused.

Done with Claude Opus 5.5 in Claude Code, running inside T3 Code.

🤖 Generated with Claude Code

[Bil0000]

Thanks for the work here, @TonybynMp4! I added your stricter parsing, GitLab work item URLs, and pull request guard to #107, with the existing preview and source project routing.

This keeps one focused stack above the original issue tracking PR. Your contribution is credited in the replacement PR. I am closing this copy so we can review and follow up in one place. Thanks again for helping improve this!