Adds a new **Enable Coder Agents** section to the Early Access doc
explaining how to activate the `agents` experiment flag via
`CODER_EXPERIMENTS` or `--experiments`.
## Changes
### `docs/ai-coder/agents/early-access.md`
- New **Enable Coder Agents** section with env var and CLI flag
examples.
- Note that the `agents` flag is excluded from wildcard (`*`) opt-in.
- Quick-start checklist: dashboard → Admin → configure provider/model →
start chatting.
- Link to GitHub issues for feedback.
### `docs/ai-coder/agents/index.md`
- Updated **Product status** from "internal preview" to "Early Access"
with a link to the early-access page for enablement instructions.
Adds a new child page at `/docs/ai-coder/agents/early-access` describing
the Coder Agents Early Access, including what it includes, what it does
not include, feature scope, licensing, and how to provide feedback.