Fix incorrect README documentation about task file matching

[Copilot]

README contained conflicting statements claiming tasks are matched by task_name frontmatter field. The actual code matches by filename (sans .md extension).

Changes

• Line 107: "Find a task file with task_name: fix-bug" → "Find a task file named fix-bug.md"
• Line 138: "<task-name> is the value of the task_name field" → "corresponds to the filename"
• Line 148: Removed incorrect reference to requiring task_name in frontmatter
• Line 168: "matching task_name in frontmatter" → "matches filename without .md extension"
• Lines 239-241: "filename doesn't matter - only task_name is used" → "matched by filename, task_name is optional metadata"

Code reference

From pkg/codingcontext/context.go:

// Match by filename (without .md extension)
baseName := strings.TrimSuffix(filepath.Base(path), ".md")
if baseName != taskName {
    return nil
}

Original prompt

There's a comment in the README that tasks are found based on frontmatter. This seems incorrect to me. Can you please check against the code?

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