Refresh the framework-owned sections of a project's AGENTS.md after a
Boundary upgrade.

`boundary new` renders AGENTS.md from a template that evolves with the
framework (new pitfalls, conventions, modules). This command re-renders the
marker-delimited blocks from the currently installed CLI's template and
splices them into the project file, leaving everything the user wrote
outside the markers untouched.

Synced blocks:
  <!-- gen:fc-is -->               FC/IS rules
  <!-- gen:naming -->              case conventions
  <!-- gen:pitfalls -->            common pitfalls
  <!-- boundary:available-modules -->  module table

NOT synced (project state): <!-- boundary:installed-modules -->. Rows for
already-installed modules are re-removed from the refreshed available
table, mirroring what `boundary add` did at install time.

Shared template helpers for the boundary CLI: template loading, {{var}}
rendering, and the module-row pattern used to keep AGENTS.md's available-
modules table in sync (`boundary add` removes a row at install time;
`boundary agents update` keeps it removed on refresh).