fix(tui): report linux clipboard copy failures

[YOMXXX]

Summary

• try available Linux clipboard helpers in order and fall back after helper failures
• surface an actionable install hint instead of treating failed clipboard writes as success
• show clipboard error messages in session copy actions

Fixes #1406

Verification

• bun test test/cli/tui/clipboard.test.ts --timeout 30000
• bun typecheck
• git diff --check

Note: the normal pre-push hook runs repo-wide bun turbo typecheck and fails in this local worktree because @typescript/native-preview-darwin-x64 is missing on disk; package-local packages/opencode typecheck passes.