Fix UTF-8 crash in evolution status reasons

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Summary

• Fixes a crash in human skilllite evolution status output when recent evolution_log.reason contains long CJK/emoji text.
• The status renderer now truncates reasons with .chars().take(...) instead of byte slicing, preserving UTF-8 boundaries with a minimal localized change.

Task Linkage

• Task ID: TASK-2026-069
• Task folder: tasks/TASK-2026-069-evolution-status-utf8-reason/

Injected Specs

• spec/architecture-boundaries.md (if architecture/layering changed)
• spec/security-nonnegotiables.md (if sandbox/security changed)
• spec/testing-policy.md (required for any code change)
• spec/docs-sync.md (if behavior/docs/env/commands changed)

Validation Evidence

• Commands executed:
  • cargo fmt --check
  • cargo clippy --all-targets -- -D warnings
  • cargo test -p skilllite-commands
  • cargo test -p skilllite-commands --features agent status_human_handles_non_ascii_event_reasons
  • cargo clippy --all-targets --all-features -- -D warnings
  • cargo test
  • python3 scripts/validate_tasks.py
• Key results:
  • Rust toolchain updated from 1.83.0 to stable 1.96.0 after the first clippy run was blocked by edition 2024 support.
  • Focused agent regression test passed: 1 passed; 0 failed.
  • Full cargo test passed.
  • Task validation passed: Task validation passed (69 task directories checked).

Regression Scope

• Areas likely affected:
  • skilllite evolution status human recent-event output only.
• Explicit non-goals:
  • No DB schema changes.
  • No desktop JSON status payload changes.
  • No backlog/proposal command behavior changes.

Docs Sync (EN/ZH)

• Not needed
• Updated EN + ZH docs
• Files:
  • N/A; this is a crash fix with unchanged command semantics.

Review Checklist

• Acceptance criteria in tasks/TASK-2026-069-evolution-status-utf8-reason/TASK.md satisfied (or explicitly deferred)
• tasks/TASK-2026-069-evolution-status-utf8-reason/STATUS.md updated with latest progress
• tasks/TASK-2026-069-evolution-status-utf8-reason/REVIEW.md includes merge readiness decision
• tasks/board.md status is up to date