feat: secure skill env setup on load (core #688)

When a skill declares required_environment_variables in its YAML
frontmatter, missing env vars trigger a secure TUI prompt (identical
to the sudo password widget) when the skill is loaded. Secrets flow
directly to ~/.hermes/.env, never entering LLM context.

Key changes:
- New required_environment_variables frontmatter field for skills
- Secure TUI widget (masked input, 120s timeout)
- Gateway safety: messaging platforms show local setup guidance
- Legacy prerequisites.env_vars normalized into new format
- Remote backend handling: conservative setup_needed=True
- Env var name validation, file permissions hardened to 0o600
- Redact patterns extended for secret-related JSON fields
- 12 existing skills updated with prerequisites declarations
- ~48 new tests covering skip, timeout, gateway, remote backends
- Dynamic panel widget sizing (fixes hardcoded width from original PR)

Cherry-picked from PR #723 by kshitijk4poor, rebased onto current main
with conflict resolution.

Fixes #688

Co-authored-by: kshitijk4poor <kshitijk4poor@users.noreply.github.com>

website/docs/user-guide/features/skills.md

@@ -116,6 +116,20 @@ metadata:
 
 Skills without any conditional fields behave exactly as before — they're always shown.
 
+## Secure Setup on Load
+
+Skills can declare required environment variables without disappearing from discovery:
+
+```yaml
+required_environment_variables:
+  - name: TENOR_API_KEY
+    prompt: Tenor API key
+    help: Get a key from https://developers.google.com/tenor
+    required_for: full functionality
+```
+
+When a missing value is encountered, Hermes asks for it securely only when the skill is actually loaded in the local CLI. You can skip setup and keep using the skill. Messaging surfaces never ask for secrets in chat — they tell you to use `hermes setup` or `~/.hermes/.env` locally instead.
+
 ## Skill Directory Structure
 
 ```