chore: add deprecation warning for eslint v9

[AmberFryar]

Description

Added deprecation warning to pepr format to alert users to upcoming breaking changes with eslint 10.
...

End to End Test:
(See Pepr Excellent Examples)

Related Issue

Fixes #

Relates to #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Other (security config, docs update, etc)

Checklist before merging

• Unit, Integration, docs, adr added or updated as needed
• Contributor Guide Steps followed

[greptile-apps]

Greptile Summary

This PR adds a deprecation warning to pepr format notifying users that Pepr v1.1.6 is the last release supporting ESLint v9, with the next release upgrading to ESLint v10.

• The migration guide URL in the warning message is still the placeholder <ADD_URL_HERE>, which will be displayed verbatim to all users running pepr format until replaced.

Confidence Score: 4/5

Not safe to merge until the placeholder URL is replaced with the actual ESLint v10 migration guide link.

The change itself is simple and correct in approach, but the placeholder URL <ADD_URL_HERE> is a P1 issue — every user running pepr format will see it verbatim, making the deprecation notice actively misleading until resolved.

src/cli/format/index.ts — line 19 needs the real migration guide URL.

Important Files Changed

Filename	Overview
src/cli/format/index.ts	Added deprecation warning for ESLint v9 support, but the migration guide URL is still a placeholder (<ADD_URL_HERE>) that will be shown to users verbatim.

Sequence Diagram

sequenceDiagram
    participant User
    participant CLI as pepr format (CLI)
    participant Log as Log.warn

    User->>CLI: runs `pepr format`
    CLI->>Log: emit deprecation warning (ESLint v9 → v10)
    CLI->>CLI: peprFormat(opts.validateOnly)
    CLI-->>User: success / exit(1)

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_{Reviews (1): Last reviewed commit: "chore: add deprecation warning for eslin..." | Re-trigger Greptile}

[greptile-apps]

Placeholder URL left in warning message

The migration guide URL is still <ADD_URL_HERE>, which will be shown verbatim to every user who runs pepr format. This should be replaced with the actual ESLint v10 migration guide URL before merging (e.g., https://eslint.org/docs/latest/use/migrate-to-10.0.0).

Suggested change

	"Review the ESLint v10 migration guide before upgrading Pepr: <ADD_URL_HERE>",
	"Review the ESLint v10 migration guide before upgrading Pepr: https://eslint.org/docs/latest/use/migrate-to-10.0.0",