feat: add PromptColor setting for customisable shell prompt colour

[MattieTK]

Summary

Adds a Set PromptColor setting that lets users change the colour of VHS's default prompt across all supported shells.

Syntax

Set PromptColor "#FF8000"    # orange prompt
Set PromptColor "#00FF00"    # green prompt

The default remains #5B56E0 (the existing blue-purple), so existing tapes are unaffected.

Motivation

Prompt customisation is one of the most requested features in VHS. Issue #419 ("How can I change Shell Prompt") has been open since mid-2023 and has driven users to alternative tools:

"I've switched to asciinema at the moment just because of this issue" – @ocervell

"This issue is basically forcing my hand to use asciinema" – @artob

"Setting PS1 with the Env command has no effect on the prompt for bash" – @artob

There was a previous attempt in PR #469 where @Delta456 tried reading os.Getenv("PROMPT") at module init time, but the environment wasn't set yet at that point. @maaslalani removed the hack and noted: "Let's remove this hack for now and keep thinking how to solve PROMPTs."

The current workaround suggested in #419 involves a Hide/Type/Enter/clear/Show sequence, which is fragile and leaves artefacts.

The colour clash problem

VHS already lets users control themes, margins, and window bars – but the prompt colour is hardcoded to #5B56E0 and cannot be adjusted. This creates two problems:

Accessibility: Themes with blue or purple backgrounds make the prompt nearly invisible. The > disappears against backgrounds of similar hue and luminance:

Set Theme "Ocean"	Set Theme "Fairyfloss"

Aesthetic clash: When users style their recordings with coloured margins or borders, the fixed purple prompt conflicts with the chosen palette:

Set MarginFill "#FF6B2B"

With Set PromptColor

Adding a single line to the tape fixes both problems:

Set Theme "Ocean"	Set Theme "Fairyfloss"
Set PromptColor "#00FF88"	Set PromptColor "#FF857A"

Set MarginFill "#FF6B2B"
Set PromptColor "#FF6B2B"

Why colour first, not the full prompt?

This PR deliberately limits scope to the prompt colour rather than the full prompt string (symbol, format, etc.). Colour is the simpler case – it doesn't affect prompt width or terminal layout, and the > symbol is consistent across shells. Full prompt symbol customisation is demonstrated in #710 (Set Prompt), which builds directly on the ShellConfig() refactoring introduced here.

How it works

1. ShellConfig(name, promptColor) replaces the old static Shells map entries. Instead of hardcoded Shell{Env: ..., Command: ...} structs, each shell's configuration is generated dynamically with the chosen colour embedded in the appropriate format (ANSI RGB for bash/osh/nushell/xonsh, hex for zsh/fish, System.Drawing.Color for PowerShell).

2. hexToRGB() parses hex colour strings (with or without #) to RGB components for shells that need numeric values.

3. Evaluator integration: Set PromptColor is processed in the same early pass as Set Shell, before the tty is started. This was the key insight that cmd: add support for ENV command #469 missed – the colour needs to be known before buildTtyCmd runs, so it's handled alongside Shell in the evaluator's pre-start loop.

Shell support

Shell	Colour format	Notes
bash	ANSI 38;2;R;G;B in PS1	RGB values from hex
zsh	%F{#RRGGBB} in PROMPT	Hex without # prefix
fish	set_color RRGGBB in fish_prompt	Hex without # prefix
PowerShell	System.Drawing.Color::FromArgb(R,G,B)	RGB values
pwsh	Same as PowerShell	Same mechanism
cmd.exe	Not supported	cmd.exe's prompt command has no colour codes; keeps default
nushell	ANSI 38;2;R;G;B in PROMPT_COMMAND	RGB values
osh	Same as bash	Shares bash's PS1 format
xonsh	ANSI 38;2;R;G;B in PROMPT	RGB values

Follow-up: Set Prompt (#710)

The ShellConfig() refactoring here also enables customising the prompt symbol. #710 demonstrates this by adding a Set Prompt setting that accepts any string:

Set PromptColor "#FF8000"
Set Prompt "$"    # bash-style

That PR adds a single parameter to ShellConfig() – a direct payoff of the structural work in this PR.

Related discussions

• How can I change Shell Prompt #419 – "How can I change Shell Prompt" – the primary motivation. Open since 2023, multiple users affected
• cmd: add support for ENV command #469 – PR that attempted prompt customisation via Env PROMPT, removed as hacky
• Escaped prompt when running VHS in a nix shell #458 – Escaped prompt in nix shell – related prompt rendering issue
• Strange prompt output at the start of the gif #39 – Strange prompt output at start of gif – prompt configuration issues on macOS

Test plan

• TestHexToRGB: hex parsing with #, without #, invalid length, empty string
• TestShellConfigReturnsNonNil: all 9 shells return valid configs
• TestShellConfigCustomColor: custom colour (#FF8000) correctly embedded in bash RGB and zsh hex
• TestShellConfigDefaultColor: default colour produces original RGB values
• TestShellConfigUnknownShell: unknown shell returns nil
• Full test suite passes (go test ./...)
• Manual testing with real tape files

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Note

This PR was authored with AI assistance (Claude Code).