Add xtab275AggressivenessHighLow, which only tags aggressiveness at high/low eagerness levels (#315043)

* Add Xtab275AggrTagHL prompting strategy

Like Xtab275Aggressiveness but only emits the <|aggressive|> tag for
high/low levels. When the level is medium, uses the plain xtab275 prompt
without any aggressiveness tag.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Rename Xtab275AggrTagHL to Xtab275AggressivenessHighLow

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Refactor: extract getXtab275BasePostScript helper

Eliminates 3x duplication of the xtab275 base postscript text across
Xtab275, Xtab275Aggressiveness, and Xtab275AggressivenessHighLow cases.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add unit tests for Xtab275AggressivenessHighLow strategy

Tests cover:
- Medium level omits the aggressive tag
- High level includes aggressive tag with 'high'
- Low level includes aggressive tag with 'low'
- High level still includes the xtab275 base postscript
- Contrast test: Xtab275Aggressiveness includes tag for medium

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* chore: trigger CI re-run

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>