ci(api-docs): isolate per-package builds and stabilize root URL

[twishabansal]

Summary

Fixes two defects in the API reference docs pipeline (go.mcp-toolbox.dev) and cleans up the generation scripts:

• Cross-package coupling: a single tag rebuilt all three packages into one shared folder, even though core, tbadk, and tbgenkit release on independent cadences. Now each per-package tag (core/v*, tbadk/v*, tbgenkit/v*) builds only that package at /<pkg>/<version>/ pushes to main build all three as //dev/.
• Root URL flapping: the root index.html was regenerated on every run, so / ping-ponged between dev and the latest tagged version. The root (repo README) is now rebuilt only on tag pushes, so it tracks the latest release and stays stable between dev builds.

URL model after this PR

Ref pushed	Builds	Root /
push to main	/core/dev/, /tbadk/dev/, /tbgenkit/dev/	untouched
tag core/v1.0.0	/core/v1.0.0/ only	rebuilt from that tag's README
any other tag	nothing (skipped)	untouched

How it was tested

Tested locally against the exact CI Hugo version (0.152.2 extended).

1. Install the CI Hugo and lint the workflow

cd /tmp && rm -rf hugo-test && mkdir hugo-test && cd hugo-test
curl -sSL -o hugo.tar.gz \
  https://github.com/gohugoio/hugo/releases/download/v0.152.2/hugo_extended_0.152.2_darwin-universal.tar.gz
tar xzf hugo.tar.gz && ./hugo version
export PATH="/tmp/hugo-test:$PATH"

cd /Users/twishabansal/mcp-toolbox-sdk-go
go run github.com/rhysd/actionlint/cmd/actionlint@latest .github/workflows/api-docs.yml   # passes clean

2. Replay the two workflow scenarios into one shared public/ tree (mimics deploying a tag on top of the existing dev site, which is how isolation gets verified)

export PATH="/tmp/hugo-test:$PATH"
rm -rf docs-site/public

# SCENARIO 1: push to main -> all three packages at dev, no root rebuild
for pkg in core tbadk tbgenkit; do
  ./scripts/generate-api-docs.sh "$pkg" dev "http://localhost:8080/"
done

# SCENARIO 2: push tag core/v1.0.0 -> only core, plus root rebuild
./scripts/generate-api-docs.sh core v1.0.0 "http://localhost:8080/"
./scripts/generate-root.sh "http://localhost:8080/"

3. Assert isolation, stable root, and content

echo "core/v1.0.0 added?       $([ -d docs-site/public/core/v1.0.0 ] && echo YES || echo NO)"
echo "core/dev still intact?   $([ -d docs-site/public/core/dev ] && echo YES || echo NO)"
echo "tbadk/dev untouched?     $([ -d docs-site/public/tbadk/dev ] && echo YES || echo NO)"
echo "tbgenkit/dev untouched?  $([ -d docs-site/public/tbgenkit/dev ] && echo YES || echo NO)"
echo "root is README?          $(grep -qiE 'Quickstart|go get' docs-site/public/index.html && echo YES || echo NO)"
echo "root NOT a redirect?     $(grep -qi 'http-equiv.*refresh' docs-site/public/index.html && echo NO || echo YES)"
echo "v1.0.0 shows 'Viewing'?  $(grep -qiE 'Viewing' docs-site/public/core/v1.0.0/index.html && echo YES || echo NO)"
echo "v1.0.0 has API symbols?  $(grep -qE 'NewToolboxClient' docs-site/public/core/v1.0.0/index.html && echo YES || echo NO)"
echo "checked-in content kept? $([ -d docs-site/content/en ] && echo YES || echo NO)"

All assertions returned the expected values (isolation preserved, root is the README with 0 meta-refresh redirects, tag page shows Viewing v1.0.0 + gomarkdoc API symbols, docs-site/content not wiped).

4. Visual spot-check

cd docs-site/public && python3 -m http.server 8080
# browse http://localhost:8080/ , /core/dev/ , /core/v1.0.0/ , /tbadk/dev/ , /tbgenkit/dev/