feat(web): open GitHub references from pull request bodies

[incognitojam]

Note

#123, GH-123 and owner/repo#123 in a pull request body are now links, the way they are on
GitHub. What each number turns out to be decides where it opens: a pull request in a project on
this machine opens as a tab beside the one being read, an issue opens in a browser, and a number
the host has nothing under is underlined in red and still opens.

GitHub's autolinked references are its own extension rather than GFM, so remark left them as they
were: every #6039 an agent or a reviewer wrote in a description, a comment or a review stayed
plain text, and following one meant retyping it into a browser.

Reading them is the easy half. Knowing what one is takes asking the host, and a body can name a
dozen — so they are asked about together, in one GraphQL document, aliased by repository and by
number. issueOrPullRequest answers which of the two each number turned out to be, which is what
decides where it opens; a cache keeps a panel of many bodies from spawning a process per body for
numbers a neighbour just resolved.

The link is addressed at /issues/{n} before any of that comes back, because the host redirects
that to /pull/{n} for a pull request — so it is already right in a browser, and resolving only
ever improves it. A reference clicked before its answer arrives follows it as written.

Everything else follows from one rule: only an answer marks a link as broken. A request that
failed on the way — rate limited, logged out, offline — leaves every reference exactly as it was,
and a null is read as nothing-there only where the host filed NOT_FOUND against it, since
FORBIDDEN is SAML or an IP allowlist standing between the reader and something they can very
likely open themselves. A host having a bad minute must not repaint a body full of good references
as mistakes.

Inert unless a surface passes the repository to read numbers against, so #2 in a conversation —
where it is far more likely to be a step than an issue — stays the plain text it reads as.

Before / after

The description of #126, which cites the stale-stack guard from #95 and the window it was built
for (#94).

Before	After

#95 resolves to a pull request and opens as a tab here; #94 resolves to an issue and opens in a
browser, since there is no issue surface to open it in.

A reference the host has nothing under keeps its link colour and takes the mark an unknown word
takes. No pull request in this repository happens to cite a number the repository lacks, so the
state below was forced on a resolved reference to photograph it:

Reading the commits

Three layers, each of which stands alone: the plugin that reads references and renders nothing
differently on its own, the request that answers them and is called by nobody, then the wiring that
turns both on.

Verification

• vp test run for the touched suites: sourceControl (147), GitManager, and the web
  markdown-github-references, chat and pullRequest suites (469).
• Targeted tsgo --noEmit for apps/web, apps/server and packages/contracts, at each of the
  three commits rather than only at the tip.
• The matching rules are GitHub's own, each checked against its renderer before being written down:
  foo#123 and #123abc are not references, (#123), #123., #123-abc and /#123 are, #0
  is not, and code spans and link labels are left alone.
• Read live against this repository in test-t3-app: references linkified in a real description,
  #95 re-addressed itself to /pull/95 and opened here, #94 stayed addressed for the browser.
  That pass is also what found two faults nothing else did — a memoized renderer holding the first
  render's answers, and a mark cancelled by .chat-markdown a { text-decoration: none }.

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Written by an agent (T3 Code, claude-opus-5).