feat(docs): add --text-color flag for write, insert, update

[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)
• Color is applied via UpdateTextStyleRequest with foregroundColor in the same batch as text insertion
• Includes ParseTextColor() and BuildColorRequest() exported helpers in docs_formatter.go
• Unit tests for named color parsing, hex parsing (3-digit, 6-digit, with/without hash), error cases, and request construction

Motivation

When multiple collaborators edit a Google Doc via the CLI, there's no way to visually distinguish who wrote what. This flag lets different users/agents write in different colors, making authorship visible at a glance in the document.

Usage

# Named colors
gog docs write <docId> --text "my note" --text-color blue
gog docs insert <docId> "annotation" --index 42 --text-color red
gog docs update <docId> --text "comment" --text-color purple

# Hex colors
gog docs write <docId> --file notes.txt --append --text-color "#FF6600"
gog docs insert <docId> "highlight" --text-color "#0A0"

Test plan

• Unit tests for ParseTextColor (named colors, hex, error cases)
• Unit tests for BuildColorRequest (range, fields, color values)
• All existing tests pass (go test ./... — 14 packages, 0 failures)
• Live tested: created Google Doc, wrote black text, appended blue text, inserted dark green hex text — all colors verified via raw API JSON

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