feat(skills): add zwift-workout skill (.zwo builder, validator, preview, publishing)

Bike-only Zwift .zwo generator with human-readable preview + confirmation loop.

- scripts/zwo_builder.py: spec<->.zwo build/validate/parse/render/TSS estimate
- scripts/zwo.py: CLI (build, from-template, validate, parse, render, templates)
- references/zwo-format.md: condensed offline format reference
- assets/example-vo2max.zwo + tests/test_zwo.py (9 tests)
- Publishes via hosting skill; files result into Knowledge Wiki

Format ground truth: h4l/zwift-workout-file-reference

skills/zwift-workout/assets/example-vo2max.zwo

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<workout_file>
+  <author>IT Guy</author>
+  <name>VO2 Max 5x3</name>
+  <description>5 x 3 min @ 115 % FTP, 3 min Pause.</description>
+  <sportType>bike</sportType>
+  <tags>
+    <tag name="vo2max"/>
+    <tag name="zwift"/>
+  </tags>
+  <workout>
+    <Warmup Duration="600" PowerLow="0.5" PowerHigh="0.75"/>
+    <IntervalsT Repeat="5" OnDuration="180" OffDuration="180" OnPower="1.15" OffPower="0.55"/>
+    <Cooldown Duration="300" PowerLow="0.75" PowerHigh="0.4"/>
+  </workout>
+</workout_file>