fix(agent-editor): create knowledge sources and prompt templates inline instead of navigating away

[esafwan]

Summary

Inside the Agent editor (/agents/:id), the agent-in-progress lives only in local react-hook-form state. Clicking "create a new knowledge source" or "New" prompt template used to call navigate() to a standalone route (/knowledge/new, /prompts/new), abandoning the in-progress agent form and taking the user out of flow entirely. Triggers already avoid this bug — TriggersTab/TriggerModal create in-context via a Dialog, no navigation. This PR extends that pattern to Knowledge and Prompt Templates.

• Knowledge: extracted KnowledgeSourceForm as a shared, presentational component now consumed by both the standalone /knowledge/new/:id route (behavior unchanged) and a new KnowledgeSourceCreateModal, opened in-context from AgentKnowledgeModal. New sources attach directly to the agent's local Knowledge state, same as linking an existing source.
• Prompt Templates: added a create-only PromptTemplateCreateModal for the agent editor's "New" button, replacing the navigate() + returnTo/localStorage round-trip with a direct in-modal create + auto-select. The standalone /prompts/new route and its other returnTo consumers (e.g. AdvancedTab's summary-prompt flow) are untouched.

Both preserve the existing "agent must be saved first" precondition, and both new modals include a dirty-close confirm guard so accidental Escape/backdrop-click doesn't silently discard input.

Scope / follow-ups

The same navigate-away-from-unsaved-agent-form pattern exists at four more entry points in the agent editor, not touched here: MCP servers, summary prompts, execution profiles, and SSH connections (all in AgentFormPage.tsx/AdvancedTab.tsx). Deferred as follow-up work using the same in-context-modal pattern established in this PR, not silently left broken.

"Automations" (from the original bug report) maps to Agent Triggers, which already uses the correct in-context pattern — no change needed there. The separate /flows Flow-builder feature (which has its own "Automation" flow type) is not currently invoked from the agent editor, so it's out of scope.

Testing

• yarn typecheck passes with no errors.
• yarn lint fails with a pre-existing environment error (minimatch/ESLint 9 config-array crash) that reproduces identically on develop — not introduced by this change.
• Reviewed via an independent code-review pass: no correctness bugs found; the standalone /knowledge/new and /prompts/new routes were diffed to confirm the extraction is byte-for-byte equivalent, not a behavior change.

Test plan

• Manually verify: open an existing agent, Knowledge tab, "create a new knowledge source" — modal opens in place, new source appears selected/linked without leaving the page.
• Manually verify: General tab, Prompt Template "New" — modal opens in place, new template is selected in the combobox without navigation.
• Manually verify nested-dialog stacking: ESC closes only the innermost create modal, not the parent picker dialog.
• Manually verify standalone /knowledge/new and /prompts/new (direct navigation, not from agent editor) still work identically.

[esafwan]

Backend CI failure (test_document_artifact_tools.py) is pre-existing on develop — reproduces identically there (e.g. https://github.com/tridz-dev/huf/actions/runs/31401212022/job/93496367383), unrelated to this frontend-only change. All frontend checks (typecheck, vitest, build) pass.