skills: harmonise fieldtest example paths to examples/fieldtest/

Mixed Underscore/Subdir notation in two files; harmonised on the
Subdir form so the dispatched agent and the skill spec agree on
where examples land.

skills/fieldtest/SKILL.md

@@ -98,7 +98,7 @@ Hand the agent the carrier (see Handoff Contract). The agent:
    compiler torture-test. Toy problems find toy bugs.
 3. For each task:
    - drafts the program in `.ailx` Surface form,
-   - saves it as `examples/fieldtest_<milestone>_<n>_<slug>.ailx`,
+   - saves it as `examples/fieldtest/<milestone>_<n>_<slug>.ailx`,
    - runs `ail check` and `ail build` and `ail run`,
    - records the experience: what the agent reached for, what it had
      to back out of, what the diagnostic said, what surprised it,

skills/fieldtest/agents/ailang-fieldtester.md

@@ -111,12 +111,12 @@ For each example, in this order:
 1. Draft the program in `.ailx` Surface form. Reach for the milestone's
    new surface where it fits naturally — but do not contort an
    example to use a feature that doesn't fit.
-2. Save as `examples/fieldtest_<milestone-short>_<n>_<slug>.ailx`,
-   e.g. `examples/fieldtest_22_1_eq_rational.ailx`.
+2. Save as `examples/fieldtest/<milestone-short>_<n>_<slug>.ailx`,
+   e.g. `examples/fieldtest/22_1_eq_rational.ailx`.
 3. Run, in this order:
    ```bash
-   ail check examples/fieldtest_<...>.ailx
-   ail build examples/fieldtest_<...>.ailx -o /tmp/ft_<n>
+   ail check examples/fieldtest/<...>.ailx
+   ail build examples/fieldtest/<...>.ailx -o /tmp/ft_<n>
    /tmp/ft_<n>
    ```
    Note: if your repo invokes `ail` as `cargo run -p ail --` instead,
@@ -140,7 +140,7 @@ The canonical form is `.ail.json`. You do NOT hand-write it. After
 each `.ailx` runs cleanly, generate the JSON via:
 
 ```bash
-ail render --json examples/fieldtest/<...>.ailx > examples/fieldtest_<...>.ail.json
+ail render --json examples/fieldtest/<...>.ailx > examples/fieldtest/<...>.ail.json
 ```
 
 (or whichever subcommand `ail --help` lists for the surface→json