Add execute.md support and per-item frontmatter sources for execute operations

[antfu]

Motivation

• Enable users to author execute operations in a simple execute.md alongside execute.yml and derive per-item operations from tracked markdown frontmatter.
• Surface parsing warnings to the CLI so users are informed about malformed or unrecognized execute.md lines.

Description

• Added EXECUTE_MD_FILE_NAME constant and EXECUTE_MD_FILE_PLACEHOLDER, and ensure execute.md is created by ensureExecuteArtifacts when missing.
• Implemented execute sources with loadExecuteSources that merges execute.yml, execute.md (via readExecuteMdFile / execute-md-parser), and per-item frontmatter-derived ops (per-item), and exposes writeRemaining for persisting progress.
• Reworked executePendingChanges to use loadExecuteSources, emit onWarning callbacks for parsing warnings, and call the provided writeRemaining to persist remaining operations.
• Added parsing utilities (parseExecuteMdLine, md reader/writer, per-item frontmatter diff logic) and tests; updated execute tests and CLI to forward warnings to printer.warn.

Testing

• Ran the unit test suite with vitest, including new and updated tests: src/execute/index.test.ts, src/execute/schema.test.ts, src/execute/sources.test.ts, src/execute/sources/* tests (execute-md-parser.test.ts, execute-md.test.ts, per-item.test.ts), and they all passed.

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Codex Task