feat(ai): invoke a project skill with / in every chat surface

[nishantmonu51]

• Typing / (or clicking the / button) opens the skill picker in every AI chat surface, not just the project chat: explore and canvas dashboard chats, the embed chat, and the developer chat.
• The picker lists the project's skills for the agent that answers the chat (analyst or developer), derived from the ChatConfig agent; the per-surface skills flag is gone.
• The developer agent now pre-invokes load_skill for skills referenced in the prompt, like the analyst. Since the developer's model only sees the current turn's tool calls, the "already loaded" check is scoped to the current invocation, so a skill referenced again in a later turn is reloaded.
• Refactors the analyst's loop into a shared loadReferencedSkills helper and adds developer-agent tests.

Follows up on #9892.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!