chore: deprecate the Resend.Cli tool

[felipefreitag]

Deprecate the internal Resend.Cli tool. The official resend-cli replaces it. Removal follows in a later release (#148).

Every run now prints a deprecation warning to stderr and points to the official CLI. Piped stdout stays clean. The --help description and examples/README.md state the same.

The next tagged release ships the last zips, with the warning baked in.

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Summary by cubic

Deprecates the internal Resend.Cli tool in favor of the official resend-cli, without breaking existing scripted usage. Previously silent runs now emit a deprecation notice to stderr; stdout remains clean.

• Prints a deprecation warning to stderr on every run and points to https://github.com/resend/resend-cli.
• Updates the command description (--help) and examples/README.md to mark deprecation.
• Next release will still ship existing binaries with the warning; removal follows in a later release.
• Migration: switch to resend-cli.

^{Written for commit bc4c2b3. Summary will update on new commits.}

[klotty]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Auto-approved: Deprecates the internal CLI tool by adding warnings to stderr and updating documentation. Existing functionality and stdout behavior remain unchanged.

Re-trigger cubic</sub>