feat: support user-defined custom commands in PR welcome message

[rnetser]

Problem

Currently, the PR welcome message only displays built-in commands (/retest, /cherry-pick, /lgtm, /approve, etc.). Some teams have external tooling, bots, or workflows that respond to custom PR comment commands (e.g., /deploy-staging, /run-integration-tests, /preview). There's no way to document these in the welcome message, so contributors either don't know about them or maintainers have to manually explain them.

Proposed Solution

Add a custom-commands configuration option (global and per-repo) that lets users define custom commands with descriptions. These are rendered in a new Custom Commands section in the PR welcome message.

Config example (in config.yaml or .github-webhook-server.yaml):

custom-commands:
  - name: deploy-staging
    description: Deploy this PR to the staging environment
  - name: run-e2e
    description: Trigger end-to-end test suite against this PR
  - name: preview
    description: Generate a preview deployment URL

Welcome message output (new section under "Available Commands"):

#### Custom Commands
* `/deploy-staging` - Deploy this PR to the staging environment
* `/run-e2e` - Trigger end-to-end test suite against this PR
* `/preview` - Generate a preview deployment URL

Key design points:

• These are documentation-only — the server renders them in the welcome message but does NOT process them as commands. External bots/tools handle them independently.
• Section is only rendered when custom-commands is configured and non-empty.
• Supports both global config and per-repo override.

Affected Files

File	Change
webhook_server/config/schema.yaml	Add custom-commands schema definition
webhook_server/libs/handlers/pull_request_handler.py	Add _prepare_custom_commands_welcome_section property, include in _prepare_welcome_comment()
examples/config.yaml	Add commented example

Done

• Add custom-commands schema to webhook_server/config/schema.yaml
• Add _prepare_custom_commands_welcome_section property to PullRequestHandler
• Include custom commands section in _prepare_welcome_comment()
• Add commented example to examples/config.yaml
• Tests for the new welcome section (present when configured, absent when not)
• Schema validation tests

[rnetser]

Closing — feature redesigned as welcome-extra-info (broader scope). New issue to follow.