chore: remove legacy snap test infrastructure after PTY migration

[jong-kyung]

Summary

Remove the obsolete infrastructure, configuration, and active documentation for the legacy snapshot suites after the PTY migration tracked in #2116 is complete.

Context

The legacy suites under packages/cli/snap-tests/ and packages/cli/snap-tests-global/ will no longer be needed after the migration.

#1723 added the legacy paths to the VS Code Vitest exclusion, and #2135 retains them while adding the PTY snapshot path. Active repository guidance such as AGENTS.md, CONTRIBUTING.md, and the PTY snapshot README also still documents the legacy suites.

Done when

• Remove the legacy path from vitest.configSearchPatternExclude without changing the PTY snapshot exclusion.
• Remove obsolete legacy test scripts, CI wiring, configuration exclusions, and migration-only tooling.
• Update active repository documentation and agent guidance to reference only the PTY snapshot suite.
• Verify that the PTY suite is the only supported CLI snapshot test path.

[jong-kyung]

@fengmk2 I accidentally set the dependency in the wrong direction. Could you update it so that #2136 is blocked by #2116, rather than blocking it? Sorry about that, and thanks! 😭

[jong-kyung]

I’ve also updated the issue description to cover all remaining legacy snapshot test infrastructure and references in the project 🙇‍♂️