feat(docs): add --text-color flag and docs color subcommand

[sixel-et]

Summary

• Adds --text-color flag to gog docs write, gog docs insert, and gog docs update commands
• Accepts named colors (red, blue, green, purple, teal, etc.) and hex colors (#RRGGBB, #RGB, RRGGBB)
• Adds new gog docs color subcommand for applying color to existing text in a document

Usage

--text-color flag (new text)

# Named color
gog docs write <docId> --text "Hello world" --append --text-color blue

# Hex color
gog docs insert <docId> --text "Note: " --text-color "#FF6600"

docs color subcommand (existing text)

# Color specific text
gog docs color <docId> "text to find" red

# Color entire paragraph containing match
gog docs color <docId> "text to find" "#a64d79" --paragraph

# Color all occurrences
gog docs color <docId> "warning" orange --all

Implementation

--text-color flag:

• ParseTextColor() in docs_formatter.go — parses named colors (15 built-in) and hex codes
• BuildColorRequest() — creates UpdateTextStyleRequest with foregroundColor
• Color applied via BatchUpdate in the same request as text insertion (InsertText + UpdateTextStyle)
• Works with --append, --replace, and index-based insertion

docs color subcommand:

• Fetches document structure via Documents.Get() to obtain correct structural indices
• Walks paragraph elements to find matching text
• Applies UpdateTextStyleRequest via BatchUpdate
• --paragraph flag colors the entire containing paragraph, not just the matched substring
• --all flag colors all occurrences, not just the first

Tests

• TestParseTextColor_NamedColors — 7 cases including case insensitivity
• TestParseTextColor_HexColors — 8 cases including 3-digit, 6-digit, no-hash, invalid
• TestBuildColorRequest — verifies range and foreground color fields
• All 14 existing test packages pass

Motivation

Collaborative document editing where multiple contributors need visually distinct text. We use this for a multi-session AI writing workflow where each contributor writes in a different color.