docs: fix documentation drift — permissions.write incorrectly documented as required

[jamesadevine]

Fixes documentation drift in README.md:

• Remove false claim that compilation fails without permissions.write for write safe outputs. The Stage 3 executor always has write access via $(System.AccessToken); permissions.write only overrides the default with an ARM-minted credential.
• Correct the Permission Combinations table. Both the 'Only read' and 'Neither (default)' rows incorrectly showed safe outputs cannot write. All rows can write; the difference is which token is used ($(System.AccessToken) vs ARM-minted).
• Add the missing �xecution-context field to the Front Matter Fields table. The field exists in the FrontMatter struct and has its own docs page but was absent from the table.

Generated by agentic workflow run 27088800996.