docs: add Agent Relay docs (#28786) - #28920

## What this adds

Two new docs pages for **Agent Relay**, a feature that connects a
supported cloud-hosted AI agent provider's hosted sessions to
self-hosted Coder workspaces. The provider's orchestration and AI
inference stay cloud-hosted; a worker process inside the workspace
executes the agent's tool calls.

- `docs/ai-coder/agent-relay/index.md` — top-level overview: what it is,
what it does, what it is and isn't, business value, current state, and
supported providers.
- `docs/ai-coder/agent-relay/cursor.md` — the Cursor-specific child
page, naming Cursor Cloud Agents and Cursor's own bring-your-own-machine
(BYOM) terminology.

Both pages are tagged `"state": ["early access"]` in the manifest and
state the feature is in **closed preview with select customers**,
pointing readers to their Coder account team or `sales@coder.com` for
access.

## Also included

- `docs/user-guides/workspace-management.md`: broadened the workspace
definition to include coding agents alongside developers.
- `docs/reference/glossary.md`: added an Agent Relay entry (alphabetized
between Agent Firewall and AI Gateway) and added it to the existing
"agent" term-disambiguation callout, since it's easy to conflate with
Coder Agents.

## Scope note for reviewers

`Agent Relay` itself (the underlying service, config, and worker
implementation) lives in a separate, non-public repository. I don't have
access to that source from this environment, so I deliberately kept both
pages high-level and didn't invent specifics like CLI flags, config file
structure, or exact command syntax. Before this goes further, someone
with access to that repo should verify the mechanics described here
(workspace-per-session lifecycle, provider pool mapping, worker
connection model) against the real implementation, and a follow-up PR
should add a step-by-step "Getting started" section once the feature
moves past closed preview to something self-serve.

## Validation

- `docs/manifest.json` is valid JSON.
- `markdownlint-cli2`: 0 issues across all changed/added Markdown files.
- `vale --no-exit`: 0 errors on every changed/added file (a handful of
pre-existing gerund-heading warnings on `workspace-management.md` and
`glossary.md` predate this branch and aren't touched here).
- `scripts/check_emdash.sh`: passes.
- Confirmed every relative link target in the new pages resolves to a
real file in the repo.

<details>
<summary>Decision log</summary>

- Dropped an "air-gap capable" claim and a "Coder AI Gateway" claim that
would have been present in an earlier draft: the integration requires
the in-workspace worker to hold an outbound connection to the cloud
provider for inference/orchestration, so true air-gapping isn't
possible, and Coder's AI Gateway isn't in the inference path for this
feature.
- Named the specific Cursor product as **Cursor Cloud Agents** and
linked Cursor's own "bring-your-own-machine (BYOM)" terminology
(`cursor.com/docs/cloud-agent/bring-your-own-machine`) rather than
inventing Coder-specific naming for the same concept.
- Kept "workspace" lowercase per the existing style guide (`workspace`
is the documented generic-concept casing, distinct from named features
like "Workspace Proxy"), while still linking the term to the Workspace
Management page.
- Used "closed preview with select customers" rather than "Preview," and
removed a blanket "not feature-complete or stable" disclaimer sentence
per explicit direction.

</details>

---

🤖 This PR was created by Coder Agents on behalf of @mattvollmer.

---------

Co-authored-by: Nick Vigilante <nickvigilante@users.noreply.github.com>
(cherry picked from commit cb6c981)