dark-mechanicus: stop inheriting terminal-default foreground

Setting text/userMessageText/customMessageText to "" means pi renders
with the terminal's default foreground color — which on many terminal
profiles is green, leaking through to the cursor (rendered via reverse-
video `\x1b[7m`), editor-typing text, autocomplete non-selected items,
and plain chat text.

Fix: introduce an `inkPurple` var (#d4c5e8, soft readable lavender,
~14:1 contrast on the #16101e background) and use it for every token
that was previously "":
  - text            ""  -> inkPurple
  - userMessageText ""  -> inkPurple
  - customMessageText "" -> inkPurple
  - mdCodeBlock     ""  -> parchment (distinct from body, stays warm)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

themes/dark-mechanicus.json

@@ -8,6 +8,7 @@
 		"bgSunken": "#0a060f",
 
 		"parchment": "#e8d9c2",
+		"inkPurple": "#d4c5e8",
 		"textMuted": "#a89379",
 		"textDim": "#6a5e50",
 		"textFaint": "#2e2621",
@@ -45,14 +46,14 @@
 		"warning": "gildedGold",
 		"muted": "textMuted",
 		"dim": "textDim",
-		"text": "",
+		"text": "inkPurple",
 		"thinkingText": "textMuted",
 
 		"selectedBg": "selectedBg",
 		"userMessageBg": "userMsgBg",
-		"userMessageText": "",
+		"userMessageText": "inkPurple",
 		"customMessageBg": "customMsgBg",
-		"customMessageText": "",
+		"customMessageText": "inkPurple",
 		"customMessageLabel": "brass",
 		"toolPendingBg": "toolPending",
 		"toolSuccessBg": "toolSuccess",
@@ -64,7 +65,7 @@
 		"mdLink": "copper",
 		"mdLinkUrl": "textDim",
 		"mdCode": "ember",
-		"mdCodeBlock": "",
+		"mdCodeBlock": "parchment",
 		"mdCodeBlockBorder": "steel",
 		"mdQuote": "textMuted",
 		"mdQuoteBorder": "brass",