Rename the enjeu content type to challenge in code #110

Context

Ocobo chose Challenge as the English name for an enjeu. The code and the content contract still say enjeu. This ticket renames the code term in this repo so code and product use one word. The website side is tracked in ocobo-revops/website (linked below).

Scope: rename

• Content type enjeu becomes challenge, and the directory enjeux/<lang>/ becomes challenges/<lang>/.
• Schema files and exports: scripts/schemas/enjeu.schema.js, ENJEU_KEYS, the enjeu fixtures under scripts/__tests__/fixtures/, and the related tests.
• Team member field enjeu (Team member expert fields: challenge, contribution, tools, previous companies #95) becomes challenge.
• Validation messages, validateRepository branches, the new-content, publish-content and translate-content skills, the Notion adapter.
• CONTEXT.md: the glossary entry becomes Challenge, with "enjeu" as the French UI word. Its Avoid list drops "challenge".
• ADR 0002 (docs/adr/0002-enjeu-pages-as-content.md): add a note on the rename rather than rewriting the decision.
• Open issue titles and bodies: spec Spec: challenge content type and Team member expert fields #91 and its tickets (Challenge content type: core schema, Stage ids and validation #94 to Authoring skills handle Team member expert fields #103).

Out of scope

• French prose. "enjeu" is an ordinary French word in blog posts, stories and jobs; those files stay unchanged.
• The six keys (rp, se, sc, cx, sy, ai) and the English slugs (sales-compensation and so on).

Acceptance criteria

• git grep -i enjeu outside French content files returns only the CONTEXT.md line naming the French UI word and the ADR 0002 rename note.
• pnpm test and pnpm validate pass.
• The website reads challenges/<lang>/ and the Team member challenge field in the same release (coordinated with the website ticket).

Timing: decided

The rename happens inside the open PRs, before they merge:

• feat(challenge): challenge schema, Stage ids and validation #109 (enjeu schema): content type, challenges/<lang>/, challenge.schema.js, CHALLENGE_KEYS, fixtures, CONTEXT.md, ADR 0002 note.
• feat(team): expert fields (challenge, contribution, tools, previous companies) #108 (expert fields): Team member field challenge; imports CHALLENGE_KEYS once feat(challenge): challenge schema, Stage ids and validation #109 is on main.

Left for this ticket after both merge: the authoring skills and the Notion adapter (if #103 has not already used the new term), and the titles and bodies of spec #91 and tickets #94 to #103.