Adds a new add_context builtin hook that renders Go text/template arguments
against hook input and joins non-blank results as additional_context. No
external dependencies (no jq, no shell) are required.
- Renders each arg as an independent text/template against the hook input
- Joins non-blank results with newlines as additional_context
- Strict: template parse and execution errors propagate and follow on_error
- Plain-text output only; no re-evaluation or command execution
- Registered alongside existing builtins in pkg/hooks/builtins
Updates schema descriptions, types.go comments, hooks reference docs,
tips guide (updates session ID tip from jq command to add_context builtin),
canonical examples/hooks.yaml, and adds standalone examples/context_hooks.yaml.
Unit and runtime test coverage included.
