docs: remove em dashes, tighten prose, and add the docs home page

[joshua-temple]

Removes every em dash from the project and tightens the documentation prose so it reads like a person wrote it.

• Docs, README, code comments, and generated workflow output no longer contain em dashes. The changelog attribution format moves from — @user to (@user), and the generator emits the same de-dashed comment the committed promote.yaml carries.
• Reworked the docs and README voice: dropped the stacked **Term** — text bullet pattern, cut marketing adjectives, and varied sentence structure.
• Adds docs/index.md and wires it into the nav, so the published Pages site has a working landing page. The site previously built without a root page and returned a 404 at the root URL.

Verification: go build ./... and go test ./... green (1019 tests), go vet clean, regenerated orchestrate.yaml/promote.yaml contain zero em dashes, and mkdocs build --strict now produces a root index.html.