[text-auditor] Fix concrete misleading and grammatical user-facing text

## Text Improvements

The following user-facing text issues were found in the repository. Each is a low-effort fix.

### 1. Misleading compile output claims a rewrite step that is currently disabled

**File:** `Makefile` (line 211)
**Current text:** `Rewriting setup action references to $(GH_AW_SETUP_ACTION_REPO)@$(GH_AW_SETUP_ACTION_REF)...`
**Suggested fix:** `Skipping setup action reference rewrite (rewrite step is disabled).`
**Why:** The next line is commented out (`# `@python3` ./scripts/rewrite_setup_action_refs.py ...`), so the current `make compile` output states an action that does not occur.

### 2. Migration guide says there is a replacement secret name when there is none

**File:** `docs/migration-guide.md` (line 7)
**Current text:** `GitHub Agent Workflows use a different secret name:`
**Suggested fix:** `GitHub Agent Workflows no longer require this secret:`
**Why:** The section describes removing `CLAUDE_CODE_OAUTH_TOKEN` and does not provide any replacement secret.

### 3. Grammar mismatch: "either" is used but only one option is listed

**File:** `docs/migration-guide.md` (line 11)
**Current text:** `If you previously used \`CLAUDE_CODE_OAUTH_TOKEN\` for authentication, you can either:`
**Suggested fix:** `If you previously used \`CLAUDE_CODE_OAUTH_TOKEN\` for authentication:`
**Why:** "either" implies multiple options, but only one bullet follows.

## Completeness Pass

- Pattern `Rewriting setup action references to` → `Makefile:211` (only occurrence)
- Pattern `different secret name` → `docs/migration-guide.md:7` (only occurrence)
- Pattern `can either:` → `docs/migration-guide.md:11` (only occurrence)

## Suggested Actions

- [ ] Update `Makefile` compile output to match actual behavior.
- [ ] Update migration-guide secret wording to reflect current authentication behavior.
- [ ] Remove the incorrect "either" phrasing in migration guidance.

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> - [x] expires on Sep 16, 2026, 1:22 PM UTC

Start with the three locations named in the issue: Makefile line 211 and docs/migration-guide.md lines 7 and 11. Review the surrounding compile and migration text, then verify the three old phrases no longer occur with the repository-wide searches described in the issue. Done means the output matches the disabled rewrite behavior and the migration guidance is grammatically and factually accurate.

Written by the indexing model from the issue text.