fix hyphen inconsistency in DM-master/DM-worker mentions

[yahonda]

What is changed, added or deleted? (Required)

DM-master and DM-worker are the actual TiDB Data Migration (DM) tool component/binary names, matching the dm-master/dm-worker binaries and dm/master, dm/worker paths. This corpus almost always hyphenates them (290 DM-master / 409 DM-worker occurrences) but 16 sites across 11 files used a space instead of a hyphen (DM master / DM worker).

Found while reviewing a Japanese translation PR that unifies katakana DMマスター/DMワーカー renderings to the literal English form (pingcap/docs#23635) — checking whether EN itself had the same kind of inconsistency.

Unifies to the dominant hyphenated form, using a word-boundary-safe substitution (\bDM master\b / \bDM worker\b) to avoid any accidental match inside a longer word.

11 files, 16 sites.

Verified markdownlint-cli2 clean on all touched files.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): found while reviewing pingcap/docs#23635

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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