docs: sort the top-level directories into five sections by what the reader wants

Every docs directory moves under 1-getting-started, 2-handbook,
4-implementation or 5-project; the options reference (section 3) comes from
the website and has no directory here. trust-boundary stays whole under the
handbook for now. web-ui, swarm and tools split between handbook and
implementation, and the command-line references move to
2-handbook/reference. Links, path references in code and nix, and the docs
index follow the moves; no page content changes.

docs/2-handbook/integrations/gitlab.md

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+# GitLab accounts
+
+Give an agent managed accounts on one or more GitLab instances: a `glab` CLI
+and `git push` over HTTPS, both authenticated by operator-supplied access
+tokens, with no manual `glab auth login`.
+
+An agent can hold any number of accounts, each under a label you choose. Two labels
+can name two instances, such as `gitlab.com` and a self-hosted one. Provisioning
+needs no per-agent nix declaration and no rebuild: the agent fetches its tokens
+from the swarm secret store itself.
+
+## Linking an account
+
+In the swarm UI, open an agent, choose **link account**, then **gitlab
+account**. Fill in a label, the instance URL (it starts as `https://gitlab.com`)
+and the token. The form shows the recommended scopes and the controller's
+answer if it refuses the input. The linked account appears in the agent's list
+of [linked accounts](../swarm/ui.md#linking-external-accounts) with a delete
+action. The UI calls the controller's API, which you can also call directly:
+
+```text
+PUT /api/hives/{hive}/agents/{agent}/gitlab-accounts/{label}
+{"url": "https://gitlab.example.org", "token": "<access token>"}
+```
+
+- `label` is lowercase letters, digits and hyphens. It names the account in the
+  store and in the account list.
+- `url` is the instance's base URL. The host must be a DNS name: an IP
+  address, `localhost` and a single-label name get a 400, because the agent
+  reaches the instance through its hive's egress proxy, which takes names
+  only. A port and a subfolder (`https://example.org:8443/gitlab`) are part of
+  the address. The path may hold letters, digits and `. _ ~ / -`; a URL with
+  user information, a query, a fragment or any other character gets a 400, so
+  every stored account is one the agent uses.
+- A plain `http://` URL works, and then the token crosses the network in
+  cleartext. Use `https://` unless the instance sits on a trusted network.
+- `token` is a GitLab personal, group or project access token. Scope it
+  `api` and `write_repository`: `glab` needs the first, and `git push` over
+  HTTPS needs the second. Use a dedicated bot account and the narrowest token
+  that does the agent's work, because the agent has passwordless sudo and can
+  act within the token's scopes.
+
+A label that holds an account already answers 409; delete it first
+(`DELETE .../gitlab-accounts/{label}`), then link again. No route hands a
+token back. `GET /api/hives/{hive}/agents/{agent}/linked-accounts` lists the
+account as kind `gitlab`, with its label and URL.
+
+Linking also allows the instance's host in the agent's egress, as a
+`link:gitlab:<label>` entry on the swarm's allowed-domains list. swarm-controller
+derives the host from the URL it stored, so nothing the agent writes can widen
+its egress. Deleting the account removes the entry.
+
+## What the agent gets
+
+An agent with a store identity (`services.hyperhive.agent.bao.addr`) has:
+
+- the `glab` command, which reads its configuration from `<state>/glab/`;
+- a git credential helper, `git-credential-hive-gitlab`, for each linked
+  instance;
+- the `hive-agent-gitlab-accounts` unit and its timer.
+
+The unit logs in to the store with the agent's own certificate, lists
+`swarm/agents/<agent>/gitlab/`, reads each account, and writes two files in
+`<state>/glab/` (mode `0600`) on boot and every two minutes, replacing a file
+only when its content changed:
+
+- `config.yml`, `glab`'s own configuration with one `hosts` entry per instance,
+  keyed by host and port, holding the token, the API protocol and the
+  subfolder. It sets `git_protocol: https`. The `host` default is the first
+  label's instance; inside a repository `glab` uses the host of its remote.
+- `gitconfig`, one `[credential "<scheme>://<host>"]` block per instance, which
+  `/etc/gitconfig` includes. The helper answers only for a host that has an
+  account, so it never touches GitHub or the swarm's forge.
+
+The unit generates both files. A deleted account leaves them on the next run, and
+a hand edit doesn't survive. `glab` keeps one token per host: when two labels
+name the same host, the first label in alphabetical order wins. If the unit
+can't read one of the accounts, it replaces nothing and retries, so the files
+keep the last complete set.
+
+The unit fails fast and restarts itself every 30 seconds for up to a day, as
+the other store fetches do, so a store that's down or sealed never holds the
+container's boot open.
+
+## Rolling out
+
+The endpoint, the store path and the agent unit are new, and each end ignores
+what it doesn't know. An old controller has no GitLab route, so no account gets
+linked. An old agent has no unit, so a stored account sits unused. A new agent
+under an old controller lists a store directory that doesn't exist and writes an
+empty `glab` configuration.