[Feat] Add support for SKILL.md

[DearVa]

Description

This PR adds support for SKILL.md. Everywhere now scans ~/.everywhere/skills and other well-known skills including ~/.agents/skills, ~/.claude/skills, ~/.codex/skills, ~/.copilot/skills, ~/.cursor/skills and ~/.gemini/skills. Future extension is supported.

This PR also introduces a friendly management page for browsing, searching, enabling and disabling loaded skills.

Loaded skills are injected into the system prompt with variable {SkillsPrompt}. Template is:

<skills>
Here is a list of skills that contain domain specific knowledge on a variety of topics.
Each skill comes with a description of the topic and a file path that contains the detailed instructions.
When a user asks you to perform a task that falls within the domain of a skill, use the 'read_file' tool to acquire the full instructions from the file URI.
<skill>
<name>[skill-name]</name>
<description>[skill-description]</description>
<file>[path-to-skill.md]</file>
</skill>
...
</skills>

Each injected prompt only costs ~200 tokens. Additional information will be loaded on demand until the read_file tool is called by the LLM. So this mechanism both saving tokens and brings lots of abilities with a rich ecosystem.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes, no api changes)
• Documentation update
• CI/CD or Build changes

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added XML documentation to any related classes

Fixed Issues

Fixes #362

[Copilot]

Pull request overview

This PR introduces a new “Skills” feature: it discovers SKILL.md files from ~/.everywhere/skills plus several well-known agent skill folders, injects enabled skills into the system prompt, and adds a UI management page for browsing/filtering/enabling skills.

Changes:

• Add skill discovery/parsing, enablement persistence, and prompt rendering via a new SkillManager/SkillSource pipeline.
• Add a new top-level “Skills” navigation page (UI + view model) with filtering, preview, and folder opening.
• Standardize monospace font usage across several markdown/code UI surfaces via a shared MonospaceFontFamily resource.

Reviewed changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/Everywhere.Core.Tests/SkillSourceTests.cs	Adds watcher/debounce behavior tests for skill directory/file changes.
tests/Everywhere.Core.Tests/SkillParserTests.cs	Adds parser tests covering frontmatter, fallbacks, and metadata flattening.
src/Everywhere.Core/Views/Pages/WebSearchEnginePage.axaml.cs	Adjusts navigation ordering to make room for the new Skills page.
src/Everywhere.Core/Views/Pages/SkillPage.axaml.cs	Adds the new Skills top-level navigation page entry.
src/Everywhere.Core/Views/Pages/SkillPage.axaml	Implements the Skills management UI (list, filters, enable toggle, markdown preview).
src/Everywhere.Core/Views/Controls/TextDifferenceSummaryView.axaml	Switches to shared monospace font resource.
src/Everywhere.Core/Views/Controls/TextDifferenceEditor.axaml	Switches to shared monospace font resource.
src/Everywhere.Core/Views/Chat/TerminalView.axaml	Switches to shared monospace font resource and removes local font resource.
src/Everywhere.Core/ViewModels/SkillPageViewModel.cs	Adds the Skills page view model: filtering, selection, refresh, copy/open actions.
src/Everywhere.Core/ValueConverters/CommonConverters.cs	Adds a converter for markdown preview binding (ObservableStringBuilder).
src/Everywhere.Core/Skills/SkillSourceRoot.cs	Defines known skill roots (Everywhere, Agents, Claude, Codex, Copilot, Cursor, Gemini).
src/Everywhere.Core/Skills/SkillSourceGroup.cs	Introduces source grouping model for UI/prompt composition.
src/Everywhere.Core/Skills/SkillSource.cs	Adds filesystem watching + debounced change aggregation for skill roots.
src/Everywhere.Core/Skills/SkillParser.cs	Implements SKILL.md parsing for frontmatter + simple metadata extraction.
src/Everywhere.Core/Skills/SkillManager.cs	Discovers skills, persists enablement overrides, builds the <skills> prompt block.
src/Everywhere.Core/Skills/SkillDescriptor.cs	Defines runtime skill model (metadata, validation, enablement change hook, diagnostics).
src/Everywhere.Core/Skills/ISkillPromptProvider.cs	Adds prompt provider interface for injecting skills into prompts.
src/Everywhere.Core/Skills/ISkillManager.cs	Adds manager interface for listing groups and triggering refresh.
src/Everywhere.Core/I18N/Strings.zh-hans.resx	Adds Skills-page localization strings (zh-hans).
src/Everywhere.Core/I18N/Strings.resx	Adds Skills-page localization strings (en).
src/Everywhere.Core/Extensions/ServiceExtensions.cs	Registers skills services + adds Skills page to navigation.
src/Everywhere.Core/Configuration/PersistentState.cs	Persists skill enablement overrides.
src/Everywhere.Core/Chat/ChatService.cs	Injects SkillsPrompt variable into system prompt rendering.
src/Everywhere.Core/Assets/Styles.axaml	Switches markdown code font to shared monospace resource.
src/Everywhere.Core/Assets/Resources.axaml	Adds shared MonospaceFontFamily resource.
src/Everywhere.Core/AI/Prompts.cs	Adds {SkillsPrompt} placeholder to the default system prompt.
src/Everywhere.Abstractions/I18N/Strings.zh-hans.resx	Adds Common_Copied localization (zh-hans) used by copy-to-clipboard UX.
src/Everywhere.Abstractions/I18N/Strings.resx	Adds Common_Copied localization (en) used by copy-to-clipboard UX.
src/Everywhere.Abstractions/Common/DynamicNotification.cs	Removes an unused using directive.

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