feat: add ANSI color output following GitHub CLI conventions

[boneskull]

Add colored terminal output to improve readability and user experience,
following the GitHub CLI Primer color guidelines.

• Add internal/style package using mgutz/ansi for ANSI colors
• Respect NO_COLOR, CLICOLOR, and TTY detection
• Apply semantic colors: green (success), red (errors), yellow (warnings),
  cyan (branches), magenta (merged), gray (muted), bold (headers)
• Add icons: ✓ (success), ✗ (failure), ! (warning)
• Update all cmd files and tree formatting with color support
• Add .claude/rules/colors.md documenting the color scheme

[boneskull]

Stack

• main
  • feat: add ANSI color output following GitHub CLI conventions #30 - branch: colors (this PR)

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Managed by gh-stack

[Copilot]

Pull request overview

Adds a centralized terminal styling layer and applies it across command output to improve readability while respecting standard color/TTY conventions.

Changes:

• Introduces internal/style with semantic styling helpers (success/error/warn/branch/merged/muted/bold) and ✓/✗/! icons.
• Threads optional styling into tree formatting and updates most cmd/* user-facing output to use the new style helpers.
• Documents the color/icon scheme in .claude/rules/colors.md.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/tree/tree.go	Adds optional *style.Style to FormatOptions and applies styling to branch display.
internal/style/style.go	New package implementing semantic color + icon helpers with NO_COLOR/TTY behavior.
cmd/unlink.go	Styles success output and branch name.
cmd/undo.go	Styles undo flow output (muted, bold, branch, warning icon).
cmd/sync.go	Styles warnings/success/dry-run output and threads style through helpers.
cmd/submit.go	Styles submit phases, PR operations output, warnings, and prompts.
cmd/orphan.go	Styles orphan success output and branch names.
cmd/log.go	Styles porcelain/table output and threads style into tree formatting.
cmd/link.go	Styles success output and branch name.
cmd/init.go	Styles init output and trunk display.
cmd/create.go	Styles create output and commit confirmation.
cmd/continue.go	Styles continue output and completion messages.
cmd/cascade.go	Styles cascade output, conflict output, and undo snapshot logging.
cmd/adopt.go	Styles adopt success output and branch/parent display.
cmd/abort.go	Styles abort warnings and completion output.
.claude/rules/colors.md	Adds documentation for the color/icon conventions and environment behavior.

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