feat(cli): add PR formatting to fern automations upgrade --json output

[devin-ai-integration]

Description

Extends fern automations upgrade --json to include a pr field with suggested PR title, body, and commitMessage. This lets the GHA upgrade action consume pre-formatted PR content directly from the CLI instead of re-deriving it — keeping all domain logic (version diffing, changelog URLs, PR formatting) in the CLI.

Changes Made

• Added pr field (PrSuggestion | null) to AutomationsUpgradeResult — includes title, body, commitMessage when there are changes; null otherwise
• Added buildPrTitle(), buildPrBody(), buildCommitMessage(), getShortGeneratorName() helper functions
• PR title/body use → (unicode) for readability in GitHub UI; commit messages use -> (ASCII) for compatibility with plain-text git tooling
• Updated JSDoc comments in cli.ts and executeAutomationsUpgrade.ts to reflect the schema
• Added changelog entry file

Testing

• 30 unit tests covering all formatting helpers, schema contract, changelog URL derivation, and edge cases
• Validated locally against fern-demo/sdk-preview-test-config with FERN_NO_VERSION_REDIRECTION=true
• Pre-commit hooks pass

Merge order

This PR must merge and publish first. Then actions PR #9 can be merged.
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Link to Devin session: https://app.devin.ai/sessions/0b0ff224ce294b938ff5c2f5aec943fd

[devin-ai-integration]

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