fix: char-boundary panic in tool cards + green up CI clippy gate

ToolCard styled the status glyph with a hardcoded byte offset 0..3, but
✓/✗/⟳ are 3-byte chars sitting at bytes 2..5 of "  {glyph}". The split
landed mid-char and panicked at render in tuie's style slicing. Compute
the glyph region as 2 + glyph.len_utf8() instead.

Then took cargo clippy -- -D warnings from 312 failures to clean:
- scoped #![allow(dead_code)] on WIP scaffolding (federation, sensorium,
  gitea_memory, model_router, session, subagent…); gate stays live on
  active code so new orphans still fail
- scoped #![allow(deprecated)] on the legacy ratatui render path, marked
  pending removal at tuie parity — no migration on code we're deleting
- declare the gui feature (forwards to tuie/gui) — the cfg was real intent
- real fixes: duplicate SaveAndGoBack arm + dead Err arm, base64::encode,
  4 unused imports, dead assignment, private-type leak, dedup'd if/else
  branches, manual clamp/strip, &PathBuf→&Path, collapsible matches

src/api/models.rs

@@ -1,3 +1,4 @@
+#![allow(dead_code)] // WIP scaffolding not yet wired
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};