Markdown renderer externalizes in-document anchors (target="_blank") and emits headings without ids, so TOC links never scroll #57971

Summary

The shared markdown base renderer wraps every link — including in-document
anchors like [Install](#install) — in
<a href="…" class="external-link" target="_blank" rel="noopener noreferrer">
(packages/ui/src/context/marked-base.tsx:6-10), and headings come out of marked's
default renderer with no id attribute. Nothing in packages/session-ui or
packages/app handles #… clicks or resolves heading slugs (a search for
startsWith("#") / anchor scrolling in those packages only matches theme/color
code), and packages/session-ui/src/components/markdown.tsx adds no ids in
post-processing either.

Consequence: a table of contents inside rendered markdown (session timeline, and
anywhere the markdown worker's output is shown) is decorative. Clicking an entry
opens a useless new tab at the app URL plus the hash on web, and on desktop the
popup is denied or produces an empty window; even after navigating, no element
matches the hash because headings carry no ids.

Repro

1. In any session, render markdown containing a TOC, e.g. an assistant answer or a
   displayed document starting with:

   ## Contents
   - [Install](#install)
   - [Usage](#usage)
   
   ## Install
   …
   ## Usage
   …

2. Click "Install" in the rendered output.

Expected: the view scrolls to the ## Install heading (the link resolves inside the
document).
Actual: a new tab/window opens at the app origin with #install appended (web) or
nothing happens (desktop); the target heading has no anchor in either case.

Root cause

• packages/ui/src/context/marked-base.tsx:9 — target="_blank" rel="noopener noreferrer" is applied unconditionally, with no branch for href values that
  start with #.
• Marked's default heading output has no id; no slug pass exists downstream
  (packages/session-ui/src/components/markdown.tsx post-processes code links and
  favicons, not headings).
• No click delegation for in-document anchors on any markdown surface, so there is
  no fallback resolution either.

Proposed fix

1. In createMarkdownBase's link renderer, skip target/rel when
   href.trim().startsWith("#") so anchors stay in-document.
2. Emit GitHub-style slug ids on headings in the renderer (lowercase, drop
   non-word/non-space punctuation, spaces to dashes, dedupe per document), giving
   anchors a real target.
3. Where markdown renders inside a scroll container, delegate clicks on #… links
   to scroll that container, with a slug-of-heading-text fallback for documents
   rendered before ids existed.

Downstream impact

The same contract was inherited by downstream apps built around OpenCode's markdown
pipeline: e.g. omnirush-ai/omnirush-gui renders every link with target="_blank"
and no heading ids, and its file-preview surface shipped with dead TOC links until
it added local click handling plus slug lookup
(vernikr/omnirush-gui@947f078). Fixing the renderer contract here would let every
surface — and downstream ports that follow it — inherit working anchors instead of
each re-implementing click delegation.