fix: materialize artifacts in the user's own temp folder - #947

## Motivation

Shell completion, editor task panels, CI scripts and coding agents need
the workspace's tasks, and how they depend on each other, without
parsing text or loading every `vite.config.*` themselves
(#320, voidzero-dev/vite-plus#2848,
voidzero-dev/vite-plus#1182). [The task graph
RFC](https://github.com/voidzero-dev/vite-task/blob/main/docs/rfcs/task-graph-json.md)
specifies `vp run --json` for this, and this PR implements it.

## Changes

- `vp run --json` prints every package and every task in the workspace,
with each task's `dependsOn` resolved to task IDs. It's indented when
stdout is a terminal and on one line otherwise. A one-element command
array prints as a string.
- Like `vp run` without a task, it loads every package's config, now
that configs load lazily (#944). A broken config in any package fails
the whole command: the error goes to stderr, and nothing goes to stdout.
A `dependsOn` cycle isn't an error.
- `--json` is exclusive, like `--last-details`.
- `$schema`: a test generates `crates/vt/schemas/task-graph.json` from
the document's Rust types with schemars, which is only a dev-dependency,
using the types' doc comments as descriptions. `artifact!` embeds that
file. Each `--json` run copies it to `task-graph_<hash>.json` in the
user's own `vp-schemas` folder in the system temp directory (#947),
unless it's already there, and `$schema` is that file's `file://` URL.
- `vtt pipe-stdout` lets an e2e step snapshot what a command prints when
its stdout isn't a terminal.
- RFC updates:
- `$schema` points to the temp-directory copy rather than to a file in
the vite-plus package.
  - A one-element command prints as a string.
- The task graph is printed even when `dependsOn` entries form a cycle.
  - `--json` loads every config.

Keeping output that a config prints while loading off stdout, which the
RFC requires, is left for later. Only vite-plus's in-process
`vite.config.*` loader can print.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Additive, read-only CLI output path with snapshot coverage; normal `vp
run` execution and caching are unchanged aside from the unrelated
seccomp cast fix.
> 
> **Overview**
> Adds **`vp run --json`**, an exclusive `run` flag (like
`--last-details`) that **does not execute tasks** and instead prints one
JSON document listing every workspace package and task, with each task's
**`dependsOn` resolved to stable task IDs** (`packages/ui#build`, with
`%`/`#` encoded in IDs). Output is **pretty-printed on a TTY** and
**compact on a pipe**; load failures go to **stderr with empty stdout**.
> 
> The JSON includes a **`$schema` `file://` URL** pointing at a draft-07
schema: Rust document types drive serialization, a **unit test
regenerates** `crates/vt/schemas/task-graph.json` via schemars,
`build.rs` embeds it, and each run **materializes** the schema into the
user-scoped temp dir (`vp-schemas`, content-hashed filename) for
editor/agent validation. **`--json` loads every package config**,
matching bare `vp run` task listing (not lazy per-package loading).
> 
> Supporting changes: new **`task_graph_json`** session module, public
**`IndexedTaskGraph::package_graph()`**, optional **`schemars`** on
`vt_str`, test helper **`vtt pipe-stdout`**, plan/e2e snapshots,
RFC/changelog updates, and a **seccomp macro fix** casting syscall
numbers to `libc::c_int` to avoid ambiguous `PartialEq` with serde_json
in the dependency graph.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
914a2ef. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>