Support opam 2.6 and default to the HTTP repository

[smorimoto]

The stable version constraint currently excludes opam 2.6.0 even though it has been released. Raise the upper bound to <2.7.0 so ordinary runs can select stable 2.6 releases, and regenerate the distributed bundle.

Change the default repository to https://opam.ocaml.org and update the README. This enables opam 2.6's archive storage optimisation for HTTP repositories; the previous Git default does not benefit from it. The existing cache key includes both the opam version and repository URLs, so this change selects a separate cache entry.

Validation:

• Formatting, lint, type checking and knip passed.
• Rebuilt all bundles and confirmed the working tree remained clean.
• Confirmed release selection chooses 2.6.0 and that the main Linux, macOS and Windows binaries have signature assets.
• Used signature-verified official binaries on macOS with isolated roots to check HTTP archive storage, repository cache regeneration, migration from 2.5.2, local Git pinning, package installation and the opam tree JSON schema.

Cross-platform integration coverage will come from the existing CI workflow. Setup-time improvements have not been benchmarked.