Pre-RC polish: ship the README in the dist, drop always-on ext requirements, un-stale the docs

[loevgaard]

Findings from a final review pass over 2.x before drafting the RC — all small and mechanical:

• .gitattributes export-ignored README.md, so a dist install from Packagist shipped without the package's documentation (while docs/UPGRADE-2.0.md did ship) — the exact gap the SDK closed on its side in 1.1 (Dependabot can't resolve your PHP dependency files #21 there). The README ships now; the dev tooling stays ignored.
• ext-hash / ext-json dropped from require: they date from when Api computed the callback HMAC and decoded JSON itself. Since the SDK took both over, src/ uses neither — and both extensions are always-enabled on PHP ≥ 8.0 anyway, so they were pure noise. ext-ctype/ext-filter (genuinely used in Details/the factory, genuinely disableable) stay.
• Doc staleness: docs/UPGRADE-2.0.md still said state logic lives in the Operations helper (deleted with SDK 1.2); the balance-writers lists in README/UPGRADE/CLAUDE.md predated Refuse to queue a capture/refund/cancel while one is still in flight #80 making Cancel and every Refund fetch — every action that touches the payment writes balance now, and the docs say so.
• One test row for OperationPendingException::assertNoneInFlight(): a newer pending operation of a type the guard ignores (an authorize made outside the gateway) must not mask a pending money operation earlier in the list — kills the one semantically meaningful escaped mutant there.

Full gate green (368 tests); analyser, validate --strict, normalize clean; composer audit no advisories.

The review verdict itself is in the conversation: no blocking findings — with this in, 2.x is RC-ready from my side.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.53%. Comparing base (4014384) to head (7d3dfb1).

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x      #85   +/-   ##
=========================================
  Coverage     98.53%   98.53%           
  Complexity      203      203           
=========================================
  Files            19       19           
  Lines           545      545           
=========================================
  Hits            537      537           
  Misses            8        8           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.