Fixes an issue where use of MiniJinja
[`include`](https://jinja.palletsprojects.com/en/stable/templates/#include)
control structure (`{% include "filename.ext" %}`) causes a render error
`template not found: tried to include non-existing template
"filename.ext"`
### Example broken diagram
``` dot
digraph ValidatePlan {
start [shape=Mdiamond, label="Start"]
exit [shape=Msquare, label="Exit"]
test_inline_prompt [label="moo" prompt="{% include 'test.tpl.md' %}"]
// ^^^^^^^^^^^^^^^^^^^^^^^^^
start -> test_inline_prompt -> exit
}
```
### Fix
The core issue was that template rendering knew the source name for
diagnostics, but did not have a loader rooted at the prompt/goal file
location. Includes therefore failed even when the included file existed
next to the rendered file. The fix adds optional loader support to
`fabro-template`, then wires workflow rendering to the existing
`FileResolver` so includes resolve relative to the file currently being
rendered.
For `fabro validate`, there was a second manifest-specific problem:
validation runs through a bundled manifest, and the manifest builder
only bundled explicit `prompt.md` / `goal.md` files, not static
MiniJinja `include` dependencies inside those files. The manifest
builder now scans prompt/goal template text for literal `{% include
"file" %}` / `{% include 'file' %}` references and bundles those files
too. Missing or unsafe include names are left for MiniJinja/runtime
validation rather than expanding scope.
(For clarity: The fix does not support variables or arrays in
`include`.)
