fix(tui): decode JSON tool envelopes and fold untrusted wrappers for display

[jkyberneees]

Symptom

Tool results from tools returning a JSON envelope (read_file, search_files, head_tail) rendered as raw JSON in the transcript, exposing encoding/json HTML-escaped prompt-injection wrapper tags:

{"content":"<untrusted_content_dd96e237… source=\"/path/to/file.go\">\n217|\n218|\tta := textarea.New(…

Root cause

odek wraps untrusted tool output in <untrusted_content_<nonce> source="…">…</…>. Tools returning JSON serialize that string with Go's encoding/json, which HTML-escapes <, >, & by default. The TUI rendered the raw envelope instead of decoding it first. Display noise only — the wrapper is a prompt-injection boundary and is not altered in any data sent back to the model.

Fix (display layer only, internal/tui)

resultPreview (used by both live tool_result events and resumed transcripts) now normalizes before render:

1. JSON envelopes: a body that parses cleanly as an object with a string content field is decoded — the real content renders, with remaining scalar metadata (e.g. total_lines) as a one-line footer. Unknown shapes (nested values, missing content, invalid JSON) render byte-identical — fail-safe, never lossy.
2. Wrapper folding: untrusted_content wrappers become a dimmed badge line ⚠ untrusted: <source> followed by the body. Both the literal tag form (live stream events) and the `` escaped form (undecoded envelopes) are recognized.

Scope: rendering only. Nothing persisted, echoed back, or forwarded is rewritten.

Tests (TDD — written red first)

• read_file-style envelope with escaped wrapper → no `` in output, badge shows the source path, body line numbers intact, total_lines footer present
• literal wrapper in a live (non-JSON) result → folded badge
• plain non-JSON output / non-envelope JSON / nested-metadata envelope → byte-identical
• malformed (truncated) envelope → rendered raw, unchanged

Verification

• make fmt && make vet && make lint — 0 issues
• make test — full -race suite green
• go build ./...