Add Mattermost incoming webhooks integration

[durw4rd]

Motivation

There is no first-party Mattermost integration in the LaunchDarkly catalog today. Mattermost is the messaging platform of choice for many organizations that have policy or regulatory reasons not to standardize on Slack — particularly EU customers, defense / public-sector teams, and developers running self-hosted chat. The existing workaround is the ld-webhook-transformer Labs project, which is useful as a Handlebars sandbox but adds operational overhead (Vercel + Neon Postgres + OAuth app) that customers don't want to take on per-team.

A native, LD-hosted Mattermost integration is the natural next step.

What this PR adds

A new integrations/mattermost/ directory containing only static artifacts:

• manifest.json — auditLogEventsHook capability, a single uri form variable for the Mattermost incoming webhook URL, three templates (default / project / environment), and a sensible defaultPolicy scoped to production-flag changes.
• templates/default.json.hbs — Mattermost-flavored Slack-compatible attachment for flag / member / segment / metric / webhook events.
• templates/project.json.hbs — project-scoped event (no environment field).
• templates/environment.json.hbs — environment-scoped event.
• assets/images/square.svg, assets/images/horizontal.svg — official Mattermost "instrument" mark and horizontal lockup, denim colorway, sourced from the Mattermost brand kit.
• README.md, LICENSE.md (Apache-2.0).

Render output

Sample render of templates/default.json.hbs against sample-context/flag/targeting-rule-update.json (via npm run preview mattermost):

```json
{
"username": "LaunchDarkly",
"attachments": [
{
"fallback": "Henrietta Powell updated the flag Demo flag in 'Production'",
"color": "#405BFF",
"title": "Demo flag",
"title_link": "https://app.launchdarkly.com/default/production/features/demo-flag",
"text": "Henrietta Powell updated the flag Demo flag in `Production`\n\n* Removed ...\n\nComment: Adjusting the targeting rules",
"fields": [
{ "title": "Project", "value": "The big project", "short": true },
{ "title": "Environment", "value": "Production", "short": true },
{ "title": "Action", "value": "updateRules", "short": true }
],
"footer": "LaunchDarkly",
"ts": 1779266887
}
]
}
```

Validation

• `npm test` → 2387 / 2387 passed, including the per-integration JSON-validity check that renders each of the three `.json.hbs` templates against `sample-context/flag/update-all-environments.json`, `sample-context/project/update.json`, and `sample-context/environment/update.json`.
• `npm run preview mattermost` produces well-formed, Mattermost-shaped JSON for all sample contexts I tested.

Things to flag for the integrations team

1. Docs page. `manifest.links.launchdarklyDocs` points at `https://launchdarkly.com/docs/integrations/mattermost\`, which doesn't exist yet. I assume the LD docs team will publish the corresponding page in tandem with merging — happy to draft a one-pager modeled on the existing Microsoft Teams / Slack integration pages if helpful.
2. Logo usage. The SVGs in `assets/images/` come straight from mattermost.com/brand-guidelines (denim colorway, no modifications). Mattermost's guidelines permit non-commercial usage and " for Mattermost"-style naming, which this manifest follows. If LD legal / partnerships would prefer to pre-coordinate with Mattermost-Inc, that's the most likely path to a future partner-claimed upgrade (changing `author` from LaunchDarkly to Mattermost and rerouting `supportEmail`).
3. `author` field. Set to `LaunchDarkly` per the community pattern used by msteams, slack, etc. Open to changing to a Labs / community attribution if that's preferred.

I'll also send a heads-up to integrations@launchdarkly.com so the docs-page workflow can start in parallel.

Out of scope (potential follow-ups)

• `buttonAction` / interactive cards — would require a Mattermost App + backend, not just incoming webhooks.
• `flagLink` / `trigger` capabilities — same.
• Localized templates — English only for v1.

Made with Cursor

[tknopp-ld]

Looks good!

[monsagri]

LGTM!

I'll make sure we enable this once it's merged in