[BUG] design-sync truncation marker points to unshipped component docs

## Environment

- Claude Code: 2.1.238
- Bundled skill: `design-sync`
- Affected module: `lib/docs.mjs`
- Pristine module SHA-256: `bb0465b065d9f29722a9453c869b70c73f85337b748d1be28800e2999be4626e`

## Problem

The bundled `/design-sync` converter caps every matched per-component documentation body at 8,000 JavaScript characters before it emits `.prompt.md`:

```js
export const DOC_BODY_CAP = 8000;

if (body.length > DOC_BODY_CAP) {
const orig = body.length;
const cut = body.slice(0, DOC_BODY_CAP).replace(/\s+\S*$/, '');
body = (cut.length > DOC_BODY_CAP - 500 ? cut : body.slice(0, DOC_BODY_CAP)) +
`\n\n_(truncated — see ${basename(path)} for full)_`;
}
```

The marker is unusable in the published artifact. The upload contract ships each component's `.prompt.md`, `.d.ts`, `.html`, and `.jsx`, but it does not ship the matched source `.md`. The generated prompt therefore tells the design agent to read a file that does not exist in the Claude Design project.

In one 379-component bundle, a count-independent marker scan found 15 affected prompts:

- Attachments
- ButtonGroup
- ChainOfThought
- Confirmation
- Conversation
- HoverCard
- InlineCitation
- InputGroup
- Message
- Popover
- Queue
- SchemaDisplay
- SearchableList
- TestResults
- Tool

Their transformed source bodies range up to 16,299 characters. Each published prompt ends around 8 KB and loses the remaining usage guidance.

## Why the cap itself deserves re-evaluation

The source comment says the design agent reads every `.prompt.md`, so one large document would crowd out the others. The generated project README instead directs the agent to read a single component prompt on demand:

```text
For a specific component, read_file("components///.prompt.md").
```

No platform file ceiling was encountered by the larger source documents. The fixed cap's rationale therefore appears inconsistent with the current on-demand read contract. The dangling marker remains a defect even if a cap is retained.

## Reproduction

After running the bundled package converter, scan the output:

```sh
rg -l '^_\(truncated — see .* for full\)_$' ds-bundle/components
```

For any result, confirm that the referenced basename is absent from the upload tree:

```sh
find ds-bundle -type f -name '.md'
```

The only matching component guidance is the already-truncated `.prompt.md`.

## Expected behavior

Truncation must never silently discard the only shipped copy of component guidance.

Any of these would satisfy the contract:

1. Keep the full per-component body in the on-demand `.prompt.md`.
2. Make the body budget configurable and allow a project to select a bounded value that preserves its complete documents.
3. If a hard cap remains, ship the full source at a stable path and make the marker point to that actual uploaded path.

The converter should also validate that every truncation marker resolves inside the final bundle.

## Related but distinct issues

- #88268 covers seven card-scaffold findings in `lib/emit.mjs` and `lib/preview-rebuild.mjs`.
- #83670 covers the separate 120-character prop JSDoc truncation in `lib/dts.mjs`.

This report concerns `lib/docs.mjs`, the per-component `.prompt.md` body, and a dangling recovery reference. The three issues should remain independently verifiable and independently removable.

Start in lib/docs.mjs and run the bundled package converter, then use the provided rg scan on ds-bundle/components to find truncation markers. Check each referenced basename against the upload tree with find. Done means component guidance is not silently lost and every truncation marker resolves to a file shipped in the final bundle.

Scritto dal modello di indicizzazione a partire dal testo della issue.