feat: one relay platforms — discover relay-capable platforms

[siddharth-bhansali]

Summary

Adds a new one relay platforms subcommand that lists every platform supporting webhook relay, with the number of event types each exposes. Previously users had to guess a platform and try relay event-types <platform> to find out whether relay was supported at all.

Also includes a small pre-existing fix: sync/index.ts was constructing new OneApi(apiKey) without passing apiBase, causing sync to always hit prod even when the CLI was configured for dev.

Closes #123

Changes

New command

# Agent mode
$ one --agent relay platforms
{"platforms":[{"platform":"airtable","eventTypeCount":3},{"platform":"attio","eventTypeCount":27}, ...]}

# Human mode
$ one relay platforms
  Platform         Event types
  ───────────────  ───────────
  airtable         3
  attio            27
  autodesk         20
  brevo            17
  ...
  stripe           224
  typeform         2
  webflow          14

  Run `one relay event-types <platform>` to see the full event list for a platform.

Calls GET /webhooks/relay/platforms (no auth required).

Files changed

• src/lib/api.ts — added listRelayPlatforms()
• src/commands/relay.ts — added relayPlatformsCommand with agent + human output
• src/index.ts — registered one relay platforms and added it to the top-level help text
• src/lib/guide-content.ts — surfaced in overview + relay topic
• skills/one/references/relay.md — added to Step 1 discovery flow + new "Discovery Commands" section
• README.md — added a one relay section (was previously undocumented)
• src/lib/sync/index.ts — pass getApiBase() to sync's OneApi instance
• package.json — 1.41.0 → 1.42.0

Test plan

• npx tsup builds cleanly
• one relay platforms --agent returns the platforms array
• one relay platforms renders the table in human mode
• one relay event-types airtable still works (no regression)
• one relay --help lists the new platforms subcommand
• Reviewer to verify docs read well in rendered GitHub preview

🤖 Generated with Claude Code

[linear]

INT-2189