feat(markdown): add tag extension

[natemoo-re]

Adds a Markdoc-style tag lexer extension to the core Markdown component. This is the foundation for rendering Seer's structured output (entity refs, artifacts) as rich, interactive embeds inline in the markdown stream instead of plain text.

Two tag syntaxes via a marked tokenizer extension registered globally on the lexer:

• Self-closing: {% ref type="issue" id="PROJ-123" /%}—inline within text
• Block: {% artifact type="root-cause" %}{"description":"..."}{% /artifact %}—standalone or inline, with a JSON body merged into value

Tag names match [\w-]+ (valid HTML element identifiers); component-specific rendering (ref, artifact, etc.) will happen in the consumer registry as a follow-up. level on the token reflects where marked parsed it (block vs inline context).

Partial tag syntax ({% ref type="issue") is stripped unconditionally in the text token handler to avoid displaying raw source text when streaming. No issues with false positives in code spans/blocks since those use the code tokens.

The Tag component slot in MarkdownComponents follows the same WithDefault pattern as every other component override. DefaultTag returns null — the registry that maps tag names to concrete components will happen in a follow-up.

Syntax

Inline ref (self-closing, no body)

The crash is caused by {% ref type="issue" id="PROJ-ABC" /%} in the auth middleware.

Block ref (with snapshot data)

{% ref type="issue" id="PROJ-ABC" %}
{"title":"NullPointerException in auth","priority":"high","count":1832,"status":"escalating"}
{% /ref %}

Artifact (always block-level)

{% artifact type="root-cause" %}
{"one_line_description":"Race condition in session refresh","five_whys":["Session token expires during concurrent refresh","Two requests hit /refresh simultaneously","No mutex on token rotation","Auth middleware assumes single-writer"]}
{% /artifact %}

Attributes go on the opening tag (type, id). Complex data goes in the JSON body. They merge into a single value object for the renderer — attrs first, then JSON body.

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.59%	93.60%	🟢 +0.01%
Typed	133,289	133,328	🟢 +39
Untyped	9,122	9,122	±0

🔍 2 new type safety issues introduced

Type assertions (as) (2 new)

File	Line	Detail
static/app/components/core/markdown/markdown.tsx	77	as ExtendedToken — token as ExtendedToken
static/app/components/core/markdown/token.tsx	71	as ExtendedToken — token as ExtendedToken

This is informational only and does not block the PR.