[eas-cli] Add eas billing:subscribe and billing:manage commands

[jonsamp]

Why

Implements the CLI half of ENG-24459: let users subscribe to an EAS plan and get a Stripe checkout link from the terminal (e.g. eas billing:subscribe starter), plus manage/cancel an existing subscription.

The www-side half is ENG-24461 (jon/eng-24461-add-stripe-link-support-to-www), which makes the stripe-auth/checkout endpoint accept plan slugs and return session.url. This PR depends on that shipping first.

How

New billing topic:

• eas billing:subscribe <plan> (starter, agent, production-plus, enterprise) — resolves the account (-a/--account, single account, or interactive picker), POSTs stripe-auth/checkout with the plan slug, and opens/prints the returned Stripe checkout URL. If the account already has a paid subscription it does not create a second one; it points the user at billing:manage (Stripe's portal handles plan changes with proration).
• eas billing:manage — POSTs stripe-auth/customer-portal and opens/prints the portal URL (change plan, update payment method, cancel).

Implementation notes:

• Plan slug → server PlanType mapping lives in the CLI; the env-specific Stripe price ids stay server-side.
• Reuses ApiV2Client; unwraps the apiv2 { data } response envelope (eas-cli's ApiV2Client returns the raw body, unlike the website client).
• Opens URLs with open instead of better-opn: better-opn runs the URL through encodeURI, which double-encodes Stripe's already-percent-encoded checkout fragment (%2F → %252F) and breaks the link. The URL is always printed as a clickable fallback.
• Added AccountQuery.getSubscriptionAsync for the already-subscribed check.

Test Plan

• Unit tests: billingClient (envelope unwrapping), subscribe (plan mapping, already-subscribed guard, free-plan handling, non-interactive/JSON output, missing-URL error), manage (portal open, verbatim URL). yarn test src/billing src/commands/billing → 10/10 pass.
• yarn typecheck, yarn lint, yarn fmt:check clean.
• Manual: ran the built CLI against a local www with the ENG-24461 changes:

  EXPO_LOCAL=true easc papeete billing:subscribe starter --account jonexpo
  

  Returns a real https://checkout.stripe.com/... URL with the fragment intact.

Note: end-to-end against staging/prod requires ENG-24461 to be deployed first; until then the endpoint returns only { id } and subscribe reports that no URL was returned.

[linear-code]

ENG-24459

[github-actions]

✅ Thank you for adding the changelog entry!

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.74%. Comparing base (4b1f785) to head (e5c8e82).

Files with missing lines	Patch %	Lines
packages/eas-cli/src/billing/resolveAccount.ts	42.11%	11 Missing ⚠️
packages/eas-cli/src/commands/billing/subscribe.ts	86.80%	7 Missing ⚠️
packages/eas-cli/src/billing/openUrl.ts	73.69%	5 Missing ⚠️
...ckages/eas-cli/src/graphql/queries/AccountQuery.ts	0.00%	3 Missing ⚠️
packages/eas-cli/src/commands/billing/manage.ts	93.94%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4007      +/-   ##
==========================================
+ Coverage   59.67%   59.74%   +0.07%     
==========================================
  Files         941      947       +6     
  Lines       41319    41459     +140     
  Branches     8675     8698      +23     
==========================================
+ Hits        24654    24766     +112     
- Misses      16571    16599      +28     
  Partials       94       94              

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