feat(tui): add configurable syntax tokens for XML/HTML tags (#6128)

[CasualDeveloper]

Problem

XML/HTML tags use hardcoded color mappings tied to unrelated tokens (error, syntaxKeyword, syntaxOperator), preventing custom themes from controlling tag colours independently.

Solution

• Add three optional theme tokens: syntaxTag, syntaxAttribute, syntaxTagDelimiter
• Extract resolution logic into theme-resolver.ts for testability
• Provide backward-compatible fallbacks to existing tokens
• Cover fallback paths with unit tests

Related Items

• Closes [FEATURE] Add configurable syntax tokens for XML/HTML tag highlighting (#6130) #6128

Notes

• Tests run locally via bun test test/cli/tui/theme.test.ts — all pass
• Schema updated in packages/web/public/theme.json
• Default theme updated in packages/opencode/src/cli/cmd/tui/context/theme/opencode.json
• AI Assistance: OpenCode + openai/gpt-5.2-codex (high)
• Testing: Unit tests + manual verification
• Review: Human operator reviewed

[Copilot]

Pull request overview

This PR adds three optional theme tokens for XML/HTML syntax highlighting, allowing custom themes to independently control tag colors instead of inheriting from unrelated tokens. The implementation maintains backward compatibility by providing fallbacks to the original hardcoded color mappings.

• Added syntaxTag, syntaxAttribute, and syntaxTagDelimiter as optional theme tokens
• Updated type definitions to support the new tokens with proper fallback handling
• Modified the opencode default theme to explicitly define colors for the new tokens

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/web/public/theme.json	Added JSON schema definitions for the three new optional syntax token properties
packages/opencode/src/cli/cmd/tui/context/theme/opencode.json	Defined explicit color values for the new tokens in the default opencode theme
packages/opencode/src/cli/cmd/tui/context/theme.tsx	Updated TypeScript types, resolution logic, and syntax rules to support the new tokens with backward-compatible fallbacks

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[CasualDeveloper]

Re-committed with valid GPG signature. Closes #6128

[CasualDeveloper]

Will continue rebasing on top of release tags as they're set.