fix(cli): use commit SHA in changelog URL for permanent links

[Swimburger]

Description

Refs companion PR: fern-api/fiddle#703 (same fix for the fiddle coordinator path)

The "See full changelog" link in auto-versioned SDK PRs used the PR branch name (prBranch) in the GitHub blob URL. After the PR is merged and the branch is deleted, this URL returns a 404. This change uses the commit SHA instead, which is permanent — it survives branch deletion and squash merges.

Changes Made

• Replaced prBranch with repository.getHeadSha() in the changelog URL construction in GithubStep.executePullRequestMode
• The resulting URL format changes from .../blob/fern-bot/2025-04-15T12-00Z/changelog.md to .../blob/<commit-sha>/changelog.md
• Added version 0.9.8 entry in versions.yml for this fix

Testing

• pnpm run check passes
• No new unit tests — the changed line is deep inside a method that orchestrates git + GitHub API calls; the URL is passed through to parseCommitMessageForPR which is already tested independently
• End-to-end validation requires deploying and triggering a real SDK generation

Review Checklist for Humans

• Verify getHeadSha() timing: Called at line 202, after push()/forcePush() (lines 179–181) but before PR creation. HEAD should match the remote commit. Does this hold in all paths?
• skipCommit (replay) path: When skipCommit=true, commitAllChanges is skipped but createReplayBranch has already committed, so HEAD should still be valid — but worth confirming.
• Preview mode: When previewMode=true, push is skipped, so the SHA won't exist on GitHub. The old prBranch URL had the same problem (branch not pushed either), so this is not a regression — but the link will 404 in either case.
• Local HEAD vs remote: Unlike the fiddle path (which creates a signed commit via GitHub API with a new SHA), this path uses standard git push, so local HEAD should always match the remote after push. Confirm no edge case breaks this assumption.

Link to Devin session: https://app.devin.ai/sessions/46ab5d2460754756b7bce40428100c9b
Requested by: @Swimburger

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[devin-ai-integration]

🟡 Catalog pin in pnpm-workspace.yaml not updated to match new generator-cli version

Per the documented version bump process in packages/generator-cli/CLAUDE.md:96-100, when making changes to generator-cli, three files must be updated together: (1) packages/generator-cli/versions.yml, (2) pnpm-workspace.yaml catalog pin, and (3) packages/cli/cli/versions.yml. This PR only updates versions.yml to 0.9.8, but pnpm-workspace.yaml:63 still pins "@fern-api/generator-cli": 0.9.4. Since generators depend on "@fern-api/generator-cli": "catalog:" (e.g. generators/base/package.json:40), they will continue resolving to 0.9.4 and won't pick up the SHA-based changelog URL fix until the catalog pin is bumped.

Prompt for agents

The version bump process documented in packages/generator-cli/CLAUDE.md requires updating three files together:

1. packages/generator-cli/versions.yml (already done — version 0.9.8)
2. pnpm-workspace.yaml — Update the catalog pin on line 63 from 0.9.4 to 0.9.8
3. packages/cli/cli/versions.yml — Add a new CLI version entry to trigger CLI publish

Without updating the catalog pin in pnpm-workspace.yaml, generators that depend on @fern-api/generator-cli via catalog: will continue resolving to 0.9.4 and will not include this fix in their Docker images.

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