fix(artifacts): auto-capture leaves a web app's own HTML pages alone

At a session's Stop every untracked .html was snapshotted as a report, so a
web project not yet committed had its index.html taken into its owner's
Artifacts. An index.html, a page under a site folder (www, public, dist,
build, src, static, templates, assets, site, _site) or one beside scripts or
stylesheets (outside the workspace root) is now a site's own page and left
alone; a standalone HTML report is still captured.

docs/API.md

@@ -3176,7 +3176,7 @@ POST   /api/artifacts/:id/assign               {userId} → {item} — admin
 What agents produced, per OWNER (`server/lib/artifacts.js`, plan: `docs/plans/artifacts/README.md`):
 
 - **page** — a claude.ai link (`https://claude.ai/…` only, `400 EBADURL`), one row per owner and link. A session's `Artifact` tool publish — or a Claude Docs `batch` that creates a doc (`container.create`; its title is the doc's name, later batches are skipped) — is captured from its PostToolUse hook (a doc's own `frame.artifactUrl`; for a publish the first `https://claude.ai/…artifact/…` in `tool_response`), owned by the session's workspace owner — or, for a session no workspace owns (a member's `claude` started from their own terminal under S3), by the member whose account sent the hook (by its connection's uid, never the body); the host account names nobody, so such a page from the host stays unowned, with `suggested_owner_id` set to the owner of the workspace whose folder holds the session's `cwd` (else, under S3, the member whose home it is).
-- **file** — `candidates` are the documents (html, md, pdf, images, csv, office…) the workspace's sessions wrote with Write/Edit/MultiEdit/NotebookEdit, inside its folder. **Captured at Stop:** when a session a workspace owns sends `Stop` or `SessionEnd`, each `.pptx`/`.docx`/`.xlsx`/`.pdf`/`.html` in the workspace it created or changed since it started (a repo's untracked or modified files — an HTML file only when untracked — or a plain folder's files; at most 20 per stop, ≤ 25 MB) is snapshotted into the workspace owner's library as `auto: true` (shown **Auto**), off the hook's response path. The same file at the mtime its snapshot recorded is skipped; a changed one replaces that row's snapshot; a file pinned by hand is left alone. A pin is read under the download route's guards (`lanePath`, `resolveInLane`, `ESECRET`, ≤ 25 MB, `413 ETOOBIG`) and SNAPSHOTTED to `<data dir>/artifacts/<random>/`.
+- **file** — `candidates` are the documents (html, md, pdf, images, csv, office…) the workspace's sessions wrote with Write/Edit/MultiEdit/NotebookEdit, inside its folder. **Captured at Stop:** when a session a workspace owns sends `Stop` or `SessionEnd`, each `.pptx`/`.docx`/`.xlsx`/`.pdf`/`.html` in the workspace it created or changed since it started (a repo's untracked or modified files — an HTML file only when untracked and not a web app's own page: never an `index.html`, one under `www`/`public`/`dist`/`build`/`src`/`static`/`templates`/`assets`/`site`/`_site`, or one beside `.js`/`.css`-type files in a folder other than the workspace root — or a plain folder's files; at most 20 per stop, ≤ 25 MB) is snapshotted into the workspace owner's library as `auto: true` (shown **Auto**), off the hook's response path. The same file at the mtime its snapshot recorded is skipped; a changed one replaces that row's snapshot; a file pinned by hand is left alone. A pin is read under the download route's guards (`lanePath`, `resolveInLane`, `ESECRET`, ≤ 25 MB, `413 ETOOBIG`) and SNAPSHOTTED to `<data dir>/artifacts/<random>/`.
 - **skill** — the caller's own `~/.claude/skills/<name>` (their member home under S3; the host's for an admin or in local mode, else `409 ENOACCOUNT`) or a workspace's `.claude/skills/<name>`. Snapshotted whole: ≤ 300 files, ≤ 5 MB, a symlink anywhere is `ESYMLINK`, a secret-looking file name (`.env`, `*.pem`, `id_rsa`, `.netrc`…, or a data file named `*token*`/`*secret*`/`*password*`/`*credential*` — the word alone does not count in Markdown, source code or images, e.g. `design-tokens.md`) `403 ESECRET`.
 - **Share again** (a skill's ⋯ menu and detail sheet, its owner, server mode) — `reshare` takes the owner's skill folder on THIS server as the new snapshot: the row's source folder, else the same-named `skills/<name>` of a plugin the owner installed at user scope (`installed_plugins.json`, the most recently updated one, never outside their `~/.claude/plugins/`), and shares the row; installers are then offered **Update**. With no copy on the server (a skill uploaded from a browser) it answers `409 ENOSOURCE` and the page asks for the skill's folder, uploaded with `id` onto that row. An upload whose skill is named like one the uploader already has (not a workspace's) also replaces that row's version rather than adding a twin.
 - **file upload** — one file from the user's computer (≤ 25 MB, `413 ETOOBIG`), snapshotted under its own base name (no folder kept).