feat: add btw_task() for running pre-formatted task files

[gadenbuie]

Summary

This PR adds btw_task(), a new function for running pre-formatted task files with YAML frontmatter configuration and markdown body content. Task files use the same format as btw.md files but describe specific workflows that can be executed interactively or as tools.

Key features:

• Template interpolation: Named arguments become template variables using {{ variable }} syntax
• Additional context: Unnamed arguments are converted to text via btw() and appended to the system prompt
• Multiple execution modes: Interactive app (default), console chat, or return as ellmer tool/client
• Flexible configuration: Task files define their own client, model, and tool configuration
• Tool metadata: Task files can specify name, title, description, and icon for tool mode

The implementation includes:

• Core btw_task() function (251 lines) with comprehensive documentation
• Test suite with 225 lines of coverage including template interpolation and context handling
• Three example task files: analyze-package.md, code-review.md, data-summary.md
• README in inst/tasks/ explaining the task file format

This establishes the foundation for user-defined task workflows that can be shared, versioned, and invoked both interactively and programmatically.

Verification

Create a simple task file and run it:

# Create a task file
tmp <- tempfile(fileext = ".md")
cat(file = tmp, '---
client: anthropic/claude-sonnet-4-6
tools: [docs]
---

Summarize the {{ pkg }} package in one sentence.
')

# Run with template interpolation
btw_task(tmp, pkg = "dplyr", mode = "tool")

# Or with additional context
btw_task(tmp, pkg = "ggplot2", mtcars, mode = "client")