Add implementation plan: agent syntax & runtime model

Covers parser (lexer + grammar + AST), runtime AgentDef/AgentFactory,
and BaseScript binding. Execution and the nf-agent plugin are deferred
to follow-up plans; AgentDef.run() throws UnsupportedOperationException
in this milestone.

Signed-off-by: Paolo Di Tommaso <paolo.ditommaso@gmail.com>

Author: pditommaso