ci: switch claude-code-action pin to upstream anthropics/claude-code-action@v1.0.104 (#8012)

## Summary

Switches the reusable Claude workflow's action pin from the personal
fork `izaitsevfb/claude-code-action@forked-pr-fix` to upstream
`anthropics/claude-code-action@b4d67413279fc18c6e5de930ae307c4f108714eb`
(v1.0.104), pinned by commit SHA for supply-chain security. The fork was
created to fix a fork-PR checkout bug that upstream now handles natively
as of v1.0.104.

## Verification

### Static analysis

`izaitsevfb/claude-code-action@forked-pr-fix` has exactly **2** commits
past its merge-base with upstream:

| commit | kind | status |
|---|---|---|
| `ececd56` (GITHUB_EVENT_NAME override for workflow_run rehydration) |
feature — part of the separate `pull_request_review_comment`
secure-trigger effort | out of scope for this pin |
| `69b53a1` (unique branch name generation for forked PRs) | bug fix —
fork PRs failed because `git fetch origin <branch>` didn't find the
fork's branch in the base repo | **fixed upstream** in
[`c3bf66d`](anthropics/claude-code-action@c3bf66d)
("handle fork PRs by fetching via refs/pull/N/head", PR
anthropics/claude-code-action#962), landed in v1.0.104. Same
`isCrossRepository` check, same `refs/pull/N/head` fetch refspec. |

Conclusion: upstream v1.0.104 is at parity with `forked-pr-fix` for the
only bug it fixes.

### Live end-to-end test (pytorch/ciforge)

- **Setup**: Added a dedicated
`.github/workflows/claude-parity-test.yml` workflow pinning
`anthropics/claude-code-action@v1.0.104` to ciforge main via
[pytorch/ciforge#475](pytorch/ciforge#475).
- **Fork PR**: Opened
[pytorch/ciforge#476](pytorch/ciforge#476) from
`izaitsevfb/ciforge:iz2/fork-parity-test` → `pytorch/ciforge:main` —
exactly the fork-PR scenario that the `forked-pr-fix` bug fix covers.
- **Workflow run**: [run
24914345416](https://github.com/pytorch/ciforge/actions/runs/24914345416)
— upstream v1.0.104 correctly detected the cross-repo PR and handled it
safely:
- Restored `.claude`, `.mcp.json`, `.claude.json`, `.gitmodules`,
`.ripgreprc`, `CLAUDE.md`, `CLAUDE.local.md`, `.husky` from
`origin/main` (PR head treated as untrusted).
- Preserved PR's sensitive paths to `.claude-pr/` for review agents (not
executed in-tree).
  - Claude agent ran 12 turns in 42s, $0.25 cost, status=success.

This is exactly the behavior the fork was built to provide, now native
in upstream.

## Risk

Low. The two branches produce equivalent behavior for fork-PR checkout;
the fork's one feature delta (`ececd56`) is tied to the
`pull_request_review_comment` trigger, which `_claude-code.yml` doesn't
use (it was deliberately excluded for security — see the existing
comment in the workflow citing github/community#55940).

## Rollback

Revert this commit — no schema / config surface change.

Author: izaitsevfb

.github/workflows/_claude-code.yml

@@ -179,7 +179,7 @@ jobs:
             console.log(`Set commit author to: ${name} <${email}>`);
 
       - name: Run Claude Code
-        uses: izaitsevfb/claude-code-action@forked-pr-fix
+        uses: anthropics/claude-code-action@b4d67413279fc18c6e5de930ae307c4f108714eb # v1.0.104
         with:
           allowed_bots: "*"
           use_bedrock: "true"