Vendor the API client into the CLI, drop the planetscale-go dependency

[no-itsbackpack]

Problem:

Every API change in the CLI required touching two repos. You had to add the new endpoint to planetscale-go, get that PR merged, tag a release, bump the version in the CLI's go.mod, and only then start the actual CLI work. The CLI is the only thing here that consumes planetscale-go, so this release dance added overhead to every feature without buying us anything.

Solution:

Copy the API client from planetscale-go (v0.175.0, the exact version we were pinned to) into this repo at internal/planetscale/ and remove the module dependency. Adding or changing an endpoint is now a normal change in this repo, done in the same PR as the feature that needs it.

Details:

• The client code is byte-for-byte the v0.175.0 release, including its tests. Only the import path changed, plus the User-Agent handling below.
• All ~290 files that imported github.com/planetscale/planetscale-go/planetscale now import github.com/planetscale/cli/internal/planetscale. The package name is unchanged so no call sites moved.
• The User-Agent header is now just pscale-cli/<version>. The old second token (planetscale-go/<version>) no longer means anything since the client version is the CLI version.
• doc/api-client.md explains the new workflow for contributors and coding agents, and AGENTS.md links to it so nobody reintroduces the two-repo process out of habit.

The planetscale-go repo itself is not touched by this PR. It still exists for external users of the Go SDK; its future is a separate conversation.

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Security impact:

None expected. No new dependencies, no auth changes, no behavior changes to requests other than the User-Agent string.

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Testing:

• go build ./... and the full go test ./... suite pass, including the vendored client's own tests.
• Built the binary and confirmed it runs.

[cursor]

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There are 4 total unresolved issues (including 3 from previous reviews).

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