fix(core): repair truncation-severed spotlight wrapper in compaction prompt

[bug-ops]

Summary

• build_compression_prompt (crates/zeph-core/src/agent/tool_execution/focus.rs) truncates each message to 500 chars before handing it to the compression LLM. Untrusted tool-result content is already spotlight-wrapped (<tool-output>/<external-data>) at write time by sanitize_tool_output — the sole sanitization point for tool output — so it is not raw/unfiltered as originally assumed by the issue.
• The blind 500-char truncation could sever the wrapper's closing tag for long untrusted content, leaving an opened-but-never-closed spotlight block in the compression prompt — the boundary marker telling the compression LLM "untrusted data ends here" would go missing exactly when it matters most.
• A single message can bundle multiple concatenated wrappers of different kinds from one multi-tool-call turn (tier_loop.rs flattens every tool result of a turn into one message; build_tool_output_source maps different tools to different wrapper kinds). The fix checks both wrapper kinds independently by open/close tag count — not by presence, and not gated on the message's aggregate trust level — so a mixed-kind batch gets repaired correctly regardless of which wrapper was severed.
• Adversarial review (2 rescope rounds) confirmed the original double-sanitize approach was incorrect (re-wrapping already-wrapped content); this PR implements the narrower, correctly-scoped fix instead.

Closes #6584

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins (14992 passed)
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• gitleaks protect --staged --no-banner --redact
• New unit tests for repair_truncated_spotlight_wrapper: severed <tool-output> close, severed <external-data> close, mixed-batch with one balanced + one severed wrapper, trust_level None/Some(0) no-op, intact sub-500-char wrapper not double-appended